Was the informal usage of "kosher" an AmE one originally?

The Jewish term kosher is also used informally to mean:

• genuine and legitimate: she consulted lawyers to make sure everything was kosher (ODO)

This usage dates back to the end of the 19th century:

• generalized sense of "correct, legitimate" is from 1896. (Etymonline)

The OED cites some early usages:

• 1896 Farmer & Henley Slang IV. 135/1 Kosh (or Kosher)... Adj. (common).—Fair; square.

• 1924 Cosmopolitan Nov. 104/2 It don't sound kosher to me!

Questions:

• Was this informal usage of "kosher" an AmE one originally?
• Is the term mainly used in AmE or is it common usage in BrE and in other English dialects?

Prelude: ‘Kosher’ in its early days as an English term

Encyclopaedia Judaica, volume 11 (2007) [combined snippets] has this note on the historical meaning of kosher (or kasher, as it was sometimes spelled in English, as well):

KASHER , or KOSHER [Hebrew spelling omitted], terms originally used in the Bible in the sense of “fit” or “proper” (e.g., Esth. 8:5; Eccles. 10:10; 11:6), and later in rabbinic literature exclusively for objects that are ritually correct and faultless. Most often it denotes food that is permitted in contrast to that which is non-kasher, or terefah. It is also used to indicate that scrolls of the Torah, tefillin, and mezuzot are properly written, that zizit are correctly spun, and that a mikveh is properly constructed. Witnesses competent to testify in accordance with Talmudic jurisprudence are also described as kasher. Recently this word has been used popularly in Anglo-Saxon countries to indicate that which is proper and within the law.

Association of the word kosher with the concept of orthodox Jewish dietary rules, or more broadly with approved forms and ritual observances of Judaism goes back in English to the first half of the nineteenth century. From “Letter from Mr. Marc, Frankfort,” in The Jewish Expositor, and Friend of Israel (February 1822):

I lately gave a copy of Mr. Wilson’s sermon to Mr. J. Shortly afterwards he requested another copy , as Mr. K. a respectable father of a family, who formerly had been a member of the School direction, had taken the first from him, and Mr. J. observed, that he had done it with a look, and with an expression of desire, that he could not think him kosher (so flesh is called when the beast has been killed according to the Jewish rites). Mr. J., added, that in his opinion, more than 20 Jews. here had the attestation of their baptism in their pockets ; he meant to say, that they had been baptized in other places, but had their reasons for keeping it yet a secret.

And from Isaac Disraeli, The Genius of Judaism (1833):

Two spirits have been conjured up in the bewitched circle—there haunts Kasher, the lawful food, and Treffo, the impure. Remove a pan, or handle a knife, and you raise that multiform demon Treffo, which no Hebrew dare touch, and whose diabolical agency is at eternal war with that benevolent spirit to hungry Jews, their beloved Kasher. This active diabolism of Treffo is occasioned by a duplicate set of culinary utensils to preserve the sanctity of the Mosaic kitchen ; those dedicated to butter must not touch those appropriated for meat.

An item in the [Lansing, Minnesota] Mower County Transcript (April 20, 1892) describes how the term kosher came to Mower County:

This word “kosher” has often been heard since the Russian Hebrews have begun to come to us in such numbers. Food and observances that are permitted are “kosher.”

Early attempts to identify ‘kosher’ as a slang term

Albert Barrere & Charles Leland, Dictionary of Slang, Jargon & Cant, volume 1 (1889), has an entry for kosh, but not for kosher:

Kosh (common), a blow as from a stick or club. From the gypsy kasht or kosh, a stick. Vide COSH.

James Maitland, The American Slang Dictionary (1891) doesn’t acknowledge kosh but does have an entry for kosher (though Maitland's definition is of the term in its traditional Hebraic sense):

Kosher (Heb., right, from yashar, to be right), pure, according to Jewish ordinances. Thus “Kosher meat” is meat killed and prepared by Jews after the Jewish manner, and so fit to be eaten by Jews.

John Farmer & William Henley, Slang and Its Analogues Past and Present, volume 4 (1896) has this interesting entry, which blurs the underworld term kosh and the Jewish term kosher:

Kosh (or Kosher), subs. (thieves’).—1. A short iron bar used for purpose of assault. 2. A blow. Adj. (common) Fair; square. {From the Hebrew = lawful.}

Farmer & Henley’s entry is the earliest dictionary definition to treat kosher as a slang term equivalent to “fair and square” or “according to Hoyle”; but the entry doesn’t cite any published instances of the word used in that sense, and instances of such usage are hard to come by before the turn of the century. In addition, the notion that kosher was a foreign borrowing may have persuaded later dictionaries not to report on its slang meaning. For example, Richard Thornton, An American Glossary: Being an Attempt to Illustrate Certain Americanisms upon Historical Principles, volume 1 (1912) doesn’t include an entry for kosher. And Mitford Matthews, A Dictionary of Americanisms on Historical Principles (1951) ignores kosher altogether.

Nevertheless kosher in the “fair and square” sense does seem to be an Americanism—just one that emerged out of a Hebrew word.

Early instances of slang 'kosher' in the wild

One early instance where kosher is used in in a way that blurs the distinction between “clean” in a religious sense and “acceptable” in a practical or secular sense appears in Israel Zangwill, Children of the Ghetto: Being Pictures of a Peculiar People, volume 2 (1892):

”You ought not to have gone there [to a Reform Sabbath service],” said Schlesinger severely. “Besides, will you deny they have the organ in their Sabbath services?”

“No, I won’t!”

“Well, then!” said De Haan, triumphantly. “If they are capable of that, they are capable of any wickedness. Orthodox people can have nothing to do with them.”

“But orthodox immigrants take their money,” said Raphael.

Their money is kosher; they are tripha," said De Haan, sententiously.

A Life magazine piece from 1897 seems to use Kosher as a synonym for “Jewish,” but since the article has certain anti-Semitic overtones, it is unclear whether that usage reflects something that might have occurred between Jews in real life in the late 1800s or is just an invention of the author’s (the latter seems somewhat more likely to me). From Metcalfe, “A First Night,” in Life (November 18, 1897):

Mr. W.: So. Vell, dot’s pusiness. Und how about dem actors. Dot Chon Drew vas a Christian. How vas it in a theatre vere everyt’ing else is Kosher der actors vas Christian?

Mr. E.: Vell, it’s like dis, you see. Charlie Frohman vas young yet and dere ain’t Kosher actors enough for all der theatres him und his zyndigate is running. You wait, though. Bimeby the actors vill be Kosher, too. Pesides, dere’s nobody for der Christian actors to vork for except Charlie und his zyndigate, so Charlie don’t haf to pay big seleries.

Mr. W.: Dot Miss Wolfe, I vos thinkin’ by her name, vos a Kosher maiden. Und she acted good, too.

Mr. E.: Yes, she acted good, but she vos not Kosher.

A similar vignette, also by Metcalfe, appears in the March 25, 1897 issue of Life.

The term kosher appears in a thoroughly figurative sense in Henry Cherouny, “Printing Trade Economics,” in The Inland Printer (February 1901):

In addition to societies of workingmen, there are politicians who require the union label to show their constituencies that they are “kosher” friends of labor. The gentlemen who, as a rule, mismanage our commonwealth, find it very cheap to curry popular favor at public expense, and union leaders think it a great achievement to introduce the union label through indirect government compulsion.

This last example seems very modern in its slangy use of kosher to mean something like “legitimate” or “credentialed” or “four-square” or perhaps “practicing/observent.”

A careful search of the Library of Congress’s Chronicling America Database for the years 1880–1900 turns up only one instance in which kosher might mean “fair and square”: The New York Sun (January 29, 1899) has a story about a dispute that arose during a game of poker, in which one player asserted that a hand consisting of the two, four, six, eight, and ten of hearts made it a “kosher mashima,” which could beat four kings. The story is told in Yiddish dialect, however, and the meaning of the term “kosher mashima” is never explained, leading me to doubt whether kosher is being used here (as in Metcalfe’s fictional dialogues) in any sense beyond “Jewish.”

J. E. Lighter, Random House Historical Dictionary of American Slang (1997) has this entry for kosher:

kosher adj. 1. genuine, correct; legitimate; honest; innocent. [Three earliest citations:] 1890–96 F[armer] & H[enley] IV 135: Kosh (or Kosher)...Adj. (common).—Fair; square. 1898 Cahan Imported Bridegroom 8: Show a treif gendarme a kosher coin, and he will be shivering with ague. Long live the American Dollar! 1901 [Irwin Sonnets (unp.) Sleep, like a bunco artist, rubbed it in,/Sold me his ten-cent oil stocks, though he knew/It was a kosher trick to take the tin.

I’ve already noted the Farmer & Henley instance, with its evident commingling of the Gypsy kosh and the Jewish kosher. The second instance, from an Abraham Cahan short story, “The Imported Bridegroom,” from his collection The Imported Bridegroom: And Other Stories of the New York Ghetto (1898) appears in the context of a conversation between an older man planning to return to the Russian village where he grew up, and his daughter:

”But the Russian police will arrest you for stayin’ away so long. Didn’t you say they would?”

”The kernel of a hollow nut!” he replied, extemporizing an equivalent of “Fiddlesticks!” Flora was used to his metaphors, although they were at times rather vague, and set to wondering how they came into his head at all. “The kernel of a hollow nut! Show a treif¹ gendarme a kosher² coin, and he will be shivering with ague. Long live the American dollar!”

_{¹Food not prepared according to the laws of Moses; impure. ²The opposite of treif.}

This example recalls the Zangwill story from 1892, where a character says that the Reform Jews are “tripha” but their money is kosher.

The third instance, which Lighter indicates is unpublished, actually appears as “Sonnet XVIII” in Wallace Irwin, The Love Sonnets of a Hoodlum (1902). Here is the six-line stanza in full:

Sleep, like a bunco artist, rubbed it in,/Sold me his ten-cent oil stocks, though he knew/It was a Kosher trick to take the tin/When I was such an easy thing to do;/For any centenarian can see/To ring a bull’s-eye when he shoots me.

I’m not at all sure what Irwin means by “Kosher” in his phrase “a Kosher trick,” but it clearly is a slang usage.

Conclusions

As JEL's excellent answer observes, kosher in its formal sense almost certainly reached English in Great Britain before it appeared in the United States. But as a slang term meaning "fair and square," "on the level," or something similar, kosher seems to have been a creation of American slang. The earliest matches for kosher that don't refer to religious requirements use the word in several ways—to mean authentic or legitimate (as "good" money is), to mean Jewish, and to mean fair and square.

One puzzle is that the definition "fair; square" that appears in Farmer & Henley in 1896 doesn't fit the way the term is used in any other Google Books match for kosher until 1901—and yet it is the meaning that survives today, and must have been in use by 1896 at the latest.

The earliest use of 'kosher' in English that I found was a variant spelling (cosher) used with the formal sense of 'purified' in 1791, in a humorous poem ("The Jews Partners and The Stolen Pork. A Tale.") by Edward Nairne, of Sandwich in Kent:

"O! Dat I ne'er de pork had sheen,
"Or ratder had a Christian been,
"For, mister Smoses, don't it eat
"Moch better dan our cosher * meat?"

*Purified

The next use of 'kosher' in English that I uncovered was an informal or at least metaphorical use in an 1822 British publication, The Jewish Expositor, and Friend of Israel: Containing Monthly Communications Respecting the Jews and the Proceedings of the London Society (Ogle, Duncan & Co.).

As I interpret this passage, Mr. J suspected that Mr. K intended to keep the copy of Mr. Wilson's sermon because Mr. K borrowed the copy of the sermon from him with a look suggesting Mr. K's intentions were not honorable ('pure' in the metaphorical, informal sense, as opposed to 'purified' in the ritual sense).

The passage is contained in a letter posted from Frankfort, Germany, to the Foreign Secretary of the London Society. The particular correspondence is undated, but the surrounding correspondence in the volume is dated 1821, and the volume itself was published in 1822.

If my interpretation of the passage shown in the clip is agreeable, the evidence contradicts claims 1-3 below, as detailed beneath each claim:

1. The earliest evidence for the use of 'kosher' in an informal sense meaning 'Correct, genuine, legitimate. colloq.' (OED Online) is from 1896 (Etymology Online, supported by the date of the earliest attestation for the informal sense given in the OED).
   A. 'Kosher' in the informal sense is attested as early as 1821.
2. 'Kosher' in the sense of 'ritually fit or proper' entered the English language in 1851 (Etymology Online, supported by the date of the earliest attestation for the formal sense given in the OED).
   B. 'Kosher' in the formal sense is attested as early as 1791.
3. The earliest evidence-based date of use of 'kosher' with an informal sense was in American English.
   C. The earliest attested use with an informal sense was in British English in 1821. The earliest attested use with a formal sense was also in British English in 1791.

Regarding your second question,

Is the term mainly used in AmE or is it common usage in BrE and in other English dialects?

all I have to offer is the opinion, based on 50-some years of extensive reading of British, US, Canadian and other literature along with frequent travel in the US and Canada, that the informal use of 'kosher' is widespread, and commonly understood by moderately literate people.

Google Ngram data, in my opinion, is not useful for the purpose of answering your second question because

1. Separating the informal from the formal uses of 'kosher' is not reasonably feasible (I'd have to look at each one, and I confess that I'm bone lazy).
2. The weighting of the collection on British as distinct from American works is unknown to me. Although the data from the "British English" corpus shows a peak around 2000 of 0.0000700%, which contrasts with the much higher American English corpus peak around 2000 of 0.000160%, I suspect that contrast might be an artifact of the data collection mechanism and procedures.