Is "or so they say" idiomatic?

I came across a long sentence followed by ellipses and the phrase "or so they say". Is the expression idiomatic?

"So they say" has been a set phrase meaning "that's what people are saying" for a long time. A Google Books search finds instances going back to Shakespeare at least. In Love's Labour's Lost, act 4, scene 4 (by 1598), this soliloquy occurs:

Biron. The king is hunting the deer, I am coursing myself. They have pitcht a toil, I am toiling in a pitch; pitch, that defiles; defile! foul word: well, set thee down, sorrow; for so they say the fool said, and so say I, the fool. Well prov'd wit.

And John Vanbrugh, A Journey to London (1730) includes it as a stand-alone sentence:

Colonel Courtly. ...Who are these Folks your Aunt has got in her House?

Colonel Martilla. One Sir Francis Headpiece and his Lady, with a Son and Daughter.

Colonel Courtly. Headpiece! Cotso, I know 'em a little. I met with 'em at a Race in the Country two Years since; a sort of Blockhead, is not he?

Colonel Martilla. So they say.

The earliest instance I could find in which "or so they say" follows a statement of supposed fact (and acts as a kind of distancing mechanism for the speaker, in case he or she might be taken to vouch for the accuracy of the reported information) occurs in "To the Ladies," in The New Mirror (October 14, 1843):

We would make a pilgrimage (if our "travels" would sell) to see the great "mother of emeralds" worshipped by the Peruvians in the valley of Manta—big as a gourd and luminous at murk midnight (or so they say).

The New Mirror was a New York periodical edited by G.P. Morris and N.P. Willis. The same periodical provides this instance from "You and One of Us" in The New Mirror (October 21, 1843):

Suppose we put the general on the gridiron and "do him brown!" Poets are so much better for toasting!—(reason why: the first lyre was made by the toasting of the sun—the tortoise shell, found by Hermes on the Nile, drawn tight by the contracted tendons—or "so they say"). His health in a glass of Elsinore cherry! And now, general, come over the coals!

And again, this instance from "Chit-Chat of New York" (dated January 18, 1844) in The New Mirror (New York, February 3, 1844):

There is more room in the city—for General Tom Thumb is departed. His littleness sailed this morning for England—"or so they say."

And again, this instance from "Confab in the Cloister," in The New Mirror (New York, April 13, 1844):

Committee.—Did you ever hear of a river in Asia called Pactolus?

Brigadier.—To be sure. An ass dipped his head into it to be able to stop making money.

Committee.—That's the fable. And ever since there have been gold sands in the river—"or so they say."

The probable source of the preceding four instances is Nathaniel Willis. At any rat three of the four instances are reprinted in The Complete Works of N. P. Willis (New York, 1846). The author's decision in each of the last three instances to place the phrase in quotation marks is striking, as it suggests that the expression might already have been a common colloquial way to frame the passing along of unconfirmed information.

Meanwhile, in England, John Kenyon uses the expression parenthetically on three separate occasions in his book, A Day at Tivoli; with Other Verses (London, 1849):

Brightening, thro' secret sympathies, the lay, / Which here he loved to weave (or so they say);

...

You scheme from town to steal away, / And chain yourself, or so they say, / To that grave joy—a wife.

...

The pictured work, with ancient graces fraught, /(Or so they say) Albertinelli wrought.

From a four-line poem titled "Honk! Honk!" in the [Sydney, New South Wales] World's News (August 16, 1919):

Money maketh the mare to go: / Or so they say, and I guess it's so; / But it's different with an auto, though— / An auto maketh the money go.

The expression "or so they say" was sufficiently familiar and idiomatic to the people of Sydney, that a columnist used it at the name of his weekly feature, according to this item in the [Sydney, New South Wales] Evening News (May 3, 1929):

"OR SO THEY SAY"

Is the title of a humorous feature, written and illustrated by Hugh M'Crae, which appears every week in the "Australian Sporting and Dramatic News."

Read It this week in the current number of the "Sporting and Dramatic," which is now on sale at all newsagents, price 6d.

Likewise in Traffic World: An Independent National Transportation Newsmagazine, volume 63 (1939) [combined snippets]:

Some of those who disagree with us and rather take the attitude that we don't know what we are talking about are railroad men who scoff at our consolidation ideas. Well, they have a right to their opinions, of course, but they should understand that they are scoffing not only at us but at many executives in their own business who believe—or say they believe—that consolidations must be effected. The railroads are even officially trying to have the consolidation parts of the interstate commerce act amended so that consolidations according to their own ideas can be more readily brought about—or so they say.

There is a degree of skepticism in this wording, as there is again in two occurrences from 1947. From Nation's Business, volume 35 (1947) [combine snippets]:

It seems so sensible EVEN the scientists in the Government cannot get together. Or so they say. They can split atoms like kindling wood but several expensive organizations may be at work doing the same thing. Or trying to do it.

And from The American Magazine, volume 143 (1947) [combined snippets]:

"But you're not interested in marriage or a home either," he pointed out. "A woman has to live by something, has to care about someone or something." He added grimly, "Or so they say."

And finally the expression appears in situations where the speaker or writer recounts an outlandish folk belief or fantastical superstition and then uses "or so they say" to disassociate himself or herself from the actual belief. From Jean Kenward, unidentified poem, in Country Life, volume 132 (1962):

Who tweaks my curtain? And what creatures are

Piping from sill and cornice? Children can

(Or so they say) encounter a green elf.

Goblins, or fairies on the naked moor:

But Nature sucks them all into herself,

And we discern only their delicate spoor

Or, on the brink of some old, hallowed spot

Hear them affront the gale—an find them not.

It seems fair to say that "or so they say" has been used idiomatically for some time as a distancing mechanism and a way to express skepticism about an assertion that one has just passed along or discussed.

Conclusions

"So they say" as a vague way of alluding to what people are saying about some topic goes back very far indeed—certainly to Shakespeare's day.

The expression "or so they say" seems qualitatively different: it doesn't merely repeat the talk of the town, but inserts a distancing "or" before passing along the rumor or report. Instances of this particular expression date back to the 1840s both in New York and in London. In early instances from both cities, the phrase is handled in a strikingly similar way—as a qualifier set in quotation marks or parentheses.

One person with a strong claim to originating "or so they say" is the U.S. editor and author Nathaniel Willis, who used it at least four times in articles published in 1843 and 1844.

But the English poet John Kenyon is another frequent user of the phrase and deserves mention as well—especially since the dates of composition of his three poems in which the phrase appears are not easy to fix with precision. He published A Day at Tivoli, with other Verses in 1849, but his previous book of poetry, Poems, for the Most Part Occasional came out in 1838, and the poems that appear in the 1849 volume might have been composed at any date after that.