Shenanigans, or shenanigan, also with several variant spellings, can be dated to 1855 USA in both the OED and Etymonline, but the OED simply says "Origin obscure" and Etymonline throws a few guesses into the ring:
Suggestions include Spanish chanada, a shortened form of charranada "trick, deceit;" or, less likely, German Schenigelei, peddler's argot for "work, craft," or the related German slang verb schinäglen. Another guess centers on Irish sionnach "fox."
Can anyone provide anything more concrete?
(Note: Another question asks specifically about “I call shenanigans” but not shenanigans itself.)
Best Answer
Wentworth & Flexner, Dictionary of American Slang (1960) suggests an extended Irish term:
The same source lists "shenanannygag" (also meaning a trick or prank) as being based on shenanigans and "not common."
Chapman & Kipfer, Dictionary of American Slang, Third Edition (1995) bump the origin to "by 1855" and retain the Wentworth/Flexner suggestion that the word perhaps comes from sionnachuighim "play tricks, be foxy."
The Facts on File Dictionary of Word and Phrase Origins (1997) begins its entry with this sentence:
I suspect that the citation in question involves a story from the California Herald cited in numerous Google Books periodicals, such as this one from the Merchants' Magazine and Commercial Review, volume 40 (1859), in which the following sentence occurs:
The earliest Google Books occurrence of this anecdote is in Yankee-Notions, vol. 5 (1856), but unfortunately the snippet view of this version of the story doesn't include the relevant term within its window. [Note (added 7 October 2015): As Hugo points out in a comment below, the relevant text from the December 1856 issue of Yankee-Notions appears in a Hathi Trust copy of that periodical.]
ADDED TO ANSWER (1/3/14):
Not surprisingly—considering its date of publication—John Russell Bartlett, Dictionary of Americanisms (1848) doesn't include an entry for shenanigan or shenanigans. But I was somewhat surprised that the second edition of this book (1859) has no such entry either. I haven't been able to find a copy of the third edition. The fourth edition (1877) does have shenanigan, along with this interesting discussion of the equivalence of shenanigan, skullduggery, and hornswoggle:
This quotation suggests that shenanigan had become popular enough in the Eastern United States by 1877 for Easterners to consider it commonplace in (and perhaps even native to) their region.
Another early discussion of shenanigan (though spelled with a fourth n) occurs in Maximilian Schele De Vere, Americanisms: The English of the New World (1872):
FURTHER UPDATE (7/30/16):
An Elephind search turns up four occurrences of shenanigan in a California newspaper between July 10, 1854, and January 1, 1855. From "By Telegraph to the Union" in the Sacramento [California] Daily Union (July 10, 1854):
From "Postscript: San Francisco Correspondence," in the Sacramento [California] Daily Union (November 16, 1854):
From "The City: Pedestrianism" in the Sacramento [California] Daily Union (December 26, 1854):
From "The City: Pastime" in the Sacramento [California] Daily Union (January 1, 1855):
The term also appears as a verb, in "Law Report: Twelfth District Court," in the [San Francisco, California] Daily Alta California (June 19, 1855):
It thus appears that shenanigan may first have been used in print in 1854 in a string of articles in the Sacramento Daily Union, where it seems consistently to have had a meaning along the lines of "fraud, deceit, or dirty trick playing."