Here is an antedating slightly older than the one posted by JEL on 12 January 2017. It too is from Springfield Globe-Republic (8 August 1886, p. 1, "The Lima Stone Man"):
“Next week, get a tent, put ‘t over his nibs, charge two jitneys to git in. Money? Well, ax me. Pard, we ain’t much on looks, but we’re some on gettin’ there.”
It may be possible to antedate the word (which has been spelled jetney, gitney, and jitney) to the 1870s:
"Does jitney mean a nickel, or a ride, or a method of transportation, or a state of mind? Apparently it now means transportation, whether it means transportation for a nickel or not is disputed. Certainly the term 'jitney,' before the jitney boom arose, meant a small coin, a cent, five cents, a dime, or a quarter. As a boy in San Francisco, the writer has heard and used the term 'jitney,' meaning either a nickel or a dime. Cents in those days were unheard of on the coast. Later, in New Orleans, the term jitney was often heard, but there it seemed to refer to a nickel alone. May we advance a theory? The jitney, we think, originated in Louisiana in the Creole word, 'jetton,' meaning a counter, or poker chip. It did not necessarily mean a nickel, because this was in the days before nickels were invented, and in Louisiana until the time of the civil war circulated a miscellaneous assortment of Spanish, French and South American coin. Therefore, jitney meant the smaller denominations of these coin—sous and pesos and what not. From Louisiana the term naturally traveled to California. It has been in use there since the days of '49. How do I know? Three reasons: first, we infer that the word was in common use because we had learned it there; secondly, because several pioneers have told us it was used; thirdly, because in reading the old files of the Overland Magazine some time ago, dated in the seventies, ]I found that the term jitney is used by one of the characters of a story of San Francisco life, the context of the story shows that this particular 'jitney' was a quarter. A further proof that a jitney is not necessarily a nickel is that in early times no coin of less value than a quarter circulated on the coast.
"So. Mr. Jitneur, when some opponent upbraids you for not being a true jitney because you may charge more than five cents, read him this article and crush him to earth. A jitney is a small coin, such as the great American public are now paying for trackless transportation.
"Postscript—Alas, for trying to prove anything! We have just received word that a Canadian board of councilmen have decided that a 'jitney' is five cents" (unsigned, "What does Jitney Mean?," The Jitney Bus, vol. 1, no. 4, July 1915, p. 114; the bracketed addition is mine).
I say "may be possible to antedate," because the passage of time (cf. "some time ago") may play tricks on one's memory. Issues of The Overland Monthly have to be examined line by line (so far I have been unable to find the word there).
The anonymous writer's speculation in the rest of the passage is likewise subject to examination. Nothing is to be taken on faith, such as his inference that "the word was in common use because we had learned it there" and his reasoning that the word had been used in California "since the days of '49 [...] because several pioneers have told us it was used."
Also, since five-cent coins have been minted in the United States since the 1790s, the writer's ability to know what the situation was "in the days before nickels were invented" is doubtful.
The following articles have more information about the word:
Gold, David L. 2009. “American English jitney 'five-cent coin; sum of five cents' Has No Apparent Jewish or Russian Connection and May Come from (Black?) Louisiana French jetnée (On the Increasing Difficulty of Harvesting All the Grain).” In his Studies in Etymology and Etiology (With Emphasis on Germanic, Jewish, Romance, and Slavic Languages). Selected and edited, with a foreword, by Félix Rodríguez González and Antonio Lillo Buades. Alicante. Publicaciones de la Universidad de Alicante. Pp.163-192, partly available here.
Gold, David L. 2018-2020. “Pursuing the origin of the American English informalism gitney ~ jitney: On the alleged Louisiana French word *jetnée and the fallacy of omne ignotum pro magnifico in etymological research.” Leuvense Bijdragen: Leuven Contributions in Linguistics and Philology. Vol. 102. Pp. 383 - 417.
This is the abstract of the second article:
The author’s first treatment of the informal American English noun gitney ~ jitney /ˈǧɪtni/ 1. ‘five-cent coin’. 2. ‘[sum of] five cents’ (see above) concludes that
No evidence supports the suggestion that the word is of Russian origin or the suggestion that it is of Yidish origin and
Evidence speaks against both suggestions.
Gold 2009 mentions another etymology suggested for gitney ~ jitney (namely, that it comes from the alleged French noun *jetnée) but does not go into detail. The present article delves deeper into that suggestion; it finds no reliable evidence for that word; and it suggests that if the word has ever existed, it is likely a reflex of gitney ~ jitney. The present article also examines Stephen Goranson’s recently suggested etymology of gitney ~ jitney, namely, < earlier American English jetney ‘?’ < French *jetnée, and finds no evidence for it.
This is the table of contents of the second article:
Introduction (with some remarks on toponymy and glottonymy)
The Financial Panic of 1914 led to the emergence of an additional meaning of the word jitney
The poem that George Washington Lee said was evidence for the alleged word *jetnée
Analysis of the version of the poem in Lee 1915
4.1. Literary texts, all the more so, poetry, may not be faithful reflections of everyday spoken usage
4.2.1. Meter and rimes
4.2.2. The conclusion of a morphological analysis of the dis legomenon *jetnée is that one of these two possibilities is right: (1) if the word has ever existed outside the poem and was coined in one or more French-related lects under no alloglossic influence, it probably meant, at least initially, *‘five sous’ worth’, *‘five centimes’ worth’, or *‘five cents’ worth’, (2) if the word has ever existed outside the poem and was coined under alloglossic influence, it is a borrowing of North American English gitney ~ jitney by speakers of one or more French-related lects, in which case it tells us nothing about the origin of that English word
4.2.3. Misprints and other mistakes
4.2.4. Use of a bare count noun in lines 1 and 3 is in all likelihood not a feature of Kouri-Vini origin
One of these two possibilities is right: (1) Lee neither coined the word *jetnée nor wrote the poem, (2) as part of a hoax, in 1915 he coined the word and wrote the poem
The fallacy of omne ignotum pro magnifico in etymological writings
In its present state, Stephen Goranson’s suggested etymology of gitney ~ jitney rests on no verified evidence
A third article on gitney ~ jitney that will propose a different French origin for the word is now in press in France and will appear in February or March 2021.
In response to Greybeard's comments (later in this thread):
Thank you for your comments.
Regarding, “‘A five-cent piece, a nickel,’” as noted in the post above edited on 29 October 2020, the word has been used to designate coins worth up to and including twenty-five American cents.
French jeton cannot be the immediate etymon of jitney for phonological reasons (an article now in press in France will give details).
The Yidish suggestion has indeed been dismissed for lack of evidence (details in Gold 2009).
There being no conceivable link been rye and a coin worth up to and including twenty-five American cents, the Ukrainian adjective житній (have you found it in Russian) cannot be the immediate etymon of American English gitney ~ jitney.
As noted above, the currently earliest-known use of jitney, however spelled, is in Springfield Globe-Republic of 8 August 1886. The number of speakers of Ukrainian in the United States before that date was tiny and their linguistic influence nil.
With respect to cobaltduck's remark (above)that "Etymonline seems to indicate the usage for coin and vehicle arose simultaneously, the vehicular sense of the word could not have arisen before 1 July 1914 (for the reason, see the section headed "The Financial Panic of 1914 led to the emergence of an additional meaning of the word jitney" (on pages 387-390 of Gold 2018-2020, referenced above). Thus, the monetary sense has to be at least twenty-eight years older than the vehicular one.