What's the etymology of the military slang word "jippo" meaning gravy?

My late grandfather who served in WWII always used to refer to gravy as jippo and it has passed down into common usage by the family, while there are sources which confirm this meaning, I cannot find any information on how the word came about.

Wikipedia lists it on the page RAF slang claiming it can also refer to stew and being another word for jollop, however searching through the results for this is even less fruitful as it seems to refer to liquor or medicine and arising from South American sources which is unlikely for an English language world war era slang term. Furthermore, my grandfather was a tank driver with the army so I question whether it's really from the RAF.

Does anyone know how or where the term "jippo" meaning gravy originated?

The word 'jippo', in the variant 'jipper', considerably predates slang use (variant 'jippo') in the RAF or World War era army. OED (paywalled) attests use in print as a transitive verb as early as 1822, in Sir Walter Scott's The Fortunes of Nigel (emphasis mine; in context, the use is as nautical slang):

This man Gregory is not fit to jipper a joint with him....

The provenance of the term becomes muddled in later years. OED attests the noun variant 'jippo' from 1929, equating it with the noun variant 'gippo', attested from 1914 and described as "slang (chiefly Services' slang)". OED gives the noun variant 'jipper' (and 'jippo') as the etymology of 'gippo'. The earliest attestation of those noun variants is for 'jipper', in 1886, when it appears in William Henry Long's A dictionary of the Isle of Wight dialect:

Jipper. Juice, or syrup of anything, as of a pudding or pie. "Mind what thee bist dooen wi' the skimmer, thee'st lat all the jipper out of the pudden."

A note in Notes and queries v.101, 1900, however, avers use of the noun variant in the sense of "gravy" around 1870:

"To JIPPER A JOINT"...It is more than thirty years ago since I sat in a Sussex chimney-corner basting thrushes suspended on worsted before a log fire. The chef de cuisine was an old naval pensioner, and his instructions were: "Mind you jipper them well." From him I also learned to call gravy "jipper," and bread-and-dripping "bread-and-jipper".

The sense of "gravy", and ascription to a nautical origin also appears in John S. Farmer and W.E. Henley, Slang and its analogues, 1896:

Jipper, subs[tantive] (nautical). — Gravy.

Earlier appearances of 'jippo' in print, in 1764 and 1806, must be regarded as semantically distinct from later uses of the noun variants 'jipper', 'gippo' and 'jippo' in the sense of "gravy" that developed after Scott's 1822 use of 'to jipper' in the sense of "to baste".

For example, in N. Bailey's 1764 An Universal Etymological English Dictionary, this definition appears:

A JIPPO, a shabby Fellow, a poor Scrub.

And in James Leslie's 1806 Dictionary of the Synonymous Words and Technical Terms in the English Language, 'jippo' appears as a synonym of the attributive 'shabby':

SHABBY. a. Shabby ragged fellow, tatterdemalion, jippo.

JEL's answer covers the earliest instances (that I'm aware of) of jipper—the probable antecedent of jippo—in the relevant sense. With regard to the etymology of jipper, I found Eric Partridge's various speculations interesting enough to merit mention here, as part of a supplemental answer.

Partridge offers this discussion of the origin of jippo in "British Soldiers' Slang with a Past," in The Quarterly Review (April 1931) [combined snippets]:

Jippo was meat-juice (especially bacon-fat or gravy) and occasionally butter. As nautical slang in 1870, jipper denoted gravy ; bread and jipper, bread and dripping ; and jipper as verb, to baste a bird or a joint of meat. In London and the Isle of Wight, in 1902, it could mean the juice or syrup of a pie, a pudding. In the modern form jippo or gippo, it is ignored by the O.E.D. The nearest I can get to an etymology is gippo, a scullion (seventeenth–eighteenth century); and that frankly a guess, although a transition from a scullion to a 'constant culinary feature' needs only to be authenticated to be declared obvious. But it is more likely that the original form was jipper, a nautical word picked up one knows not where, and that the gippo-jippo form is due to the frequent use of gyppo (gippo), jippo, as variants of gyppy, gippy, gippie for an Egyptian.

Partridge returned to the topic in his, A Dictionary of Slang and Unconventional English, second edition (1938). Here is the complete entry for jippo in that dictionary:

jipper, jippo. Gravy: nautical: from ca. 1850. Occ[asionally] it = juice, syrup, or even dripping ([Joseph Wright, The] E[nglish] D[ialect] D[ictionary]) In the C. 20 British Army, jippo, and among the Australian occ[asionally] = stew. A correspondent remembers it being , ca. 1905 at school, used of the slimy outside of pudding. Perhaps ultiamtely ex † jippo a tunic, ? hence a scullion. Just possibly, a Gippo being a man of brown colour, ex sense 2; but I shouldn't be surprised if it were proved to be a corruption of sipper.—2. Jippo, an incorrect form of Gyppo, an Egyptian.

Partridge's entry for sipper in the same dictionary is as follows:

sipper. Gravy: low: late C. 19–20. ? ex dial[ectal] sipper-sauce (ex C. 16–17 S[tandard] E[nglish] sibber-sauce), sauce, influenced by to sip.

And here is Partridge's entry for Gip:

Gip; gen. Gippo, Gyp{p}o. A gipsy: C. 20—2. Same as Gippy, 1: military: C. 20—3. (Also gypoo.) Grease; gravy; butter: military: from ca. 1912. Ex dial[ectal] gipper or jipper, meat juice, gravy. (O[xford] E[nglish] D[ictionary] Supp[lement].)

Evidently, Partridge believes that jippo in the sense of "gravy" or "juice" may ultimately be attributable to the term sibber-sauce. The term is certainly old. A.G. Dickens, Lollards and Protestants in the Diocese of York, 1509-1558 (1982) cites testimony in a 1543 heresy investigation of a 22-year-od Yorkshire shearman named William Bull, in which Bull is alleged to have made certain awful declarations the year before:

At the same time and place, Barkstone [a witness] had also heard Bull say 'that the anunctement {last unction} ys but a sybertie sawce, and that he [Bull] wolde have no suche sibertie sawce mynistred unto him at his death' and 'that he belevid in God, Father Almightie, maker of heven and earth and Jesu Christe his onely Sonne our Lorde, by whome he trustyd to be saved, yf he had no suche sybertie sawce at his death'.

Dickens adds this in a footnote:

'Sibber sauce' was later metaphorically used by Tudor protestant controversialists to represent the supposedly false garnishings of the Romanist cause. ...

Even earlier is this occurrence in William Tyndale, "The Parable of the Wicked Mammon" (1528/1536), reprinted in Elizabeth Bell Canon, The Use of Modal Expression Preference as a Marker of Style and Attribution: The Case of William Tyndale and the 1533 English 'Enchiridion Militis Christiani' (2010):

And it setteth the soul at liberty, and maketh her free to follow the will of God, and is to to the soul even-as health is unto the body of a man that is pined and wasted away with a long soaking disease. The legs cannot bear him, he cannot lift up his hands to help himself, his taste is corrupt, sugar is bitter in his mouth, his stomach abhoreth, longing after sibber sauce at the which a whole stomach is ready to cast his gorge.

But "sibber sauce" also appears in a 1615 instruction to the jury by Lord Coke, recorded in "Trials of the Murderers of Sir Thomas Overbury," in Cobbett's Complete Collection of State Trials, volume 2 (1809), with the evident meaning "poison":

That albeit the poisoning in the indictment be said to be with Rosalger, White Arsenick, and Mercury Sublimate, yet the Jury were not to expect precise proof in that point, shewing how impossible it were to convict a poisoner, who useth not to take any witnesses to the composing of his sibber sauces; wherefore he [Coke] declared the law in the like case; s if a man be indicted for murdering a man with a dagger, and it fall out upon evidence to have been done with a sword or with a rapier, or with neither, but with a staff: in this case the instrument skilleth not, so that the Jury find the murder.

In response to a query from a correspondent about this instance of "sibber sauce," the editor of Notes and Queries (June 4, 1864) offers the following etymological note:

In Scottish and in old English, sib, sibb, or sibbe, signifies related, or near of kin. We find also the comparative sibber. It would seem, however, that in speaking ironically of certain poisons as "sibber sauces," the learned lord meant "quieting sauces," i.e. sauces that quiet the partaker, or settle him. Sax. sibrum, pacific, quieting; sibbian, to pacify.

Joseph Wright, The English Dialect Dictionary, volume 2 (1900) and volume 5 (1904) includes entries for jipper, sibber-sauce, and sipper-sauce, as follows:

JIPPER, sb. and v. Lon[don and] I[sle of] W[ight] Slang. 1. sb. Gravy; juice or syrup, as of a pie or pudding. [Examples omitted.] 2. v. To baste a joint of meat. [Example omitted.]

...

SIBBER-SAUCE, sb. nw.Der[byshire]. A sauce or other dainty, used to give a relish to one's food. Cf. sipper-sauce. [Wright also notes that sibber, in Gloucestershire, means "to simmer."

...

SIPPER-SAUCE, sb. Y[or]ks[hire] 1. A sauce or other concoction taken as a relish for food; a dainty dish. Cf. sipperty ["A sauce, &c. taken as a relish with food"]. [Examples omitted.] 2. Fig[urative]. An extravagance; a superfluity.

I haven't read any professional assessments of Partridge's suggested connection between jipper and sibber/sipper. In the absence of a strong countervailing theory, anything is possible I suppose—including the possibility that what's jipper for the goose may be sipper for the gander.

In a much more recent treatment of jippo, Jonathon Green, Chambers Slang Dictionary (2008) parks his main entry under the spelling gypo:

gypo n. (also gippo, gyp, gyppo, gyppy, jippo) {abbr.; note UK services sl[ang] gyppo, gravy, grease, stew; S[outh] Afr[ica] milit[ary] gyppo, to shirk duty} 1 {late 19C+} gypsy, usu[ally] derog[atory] 2 {late 19C+} Egyptian. 3 {1920s–70s} (US) contract work, a sub-contractor, a piece-worker {the implication is that the worker fulfils the contract then moves on, like a gypsy}. 4 {1990s+} (UK juv[enile]) an impoverished, badly dressed schoolchild.

Regrettably, the etymological origin of jippo in the sense of "gravy" seems not to have piqued Green's interest.