You’ll most commonly encounter this word in the plural. It means trickery, underhand action, intrigue or skulduggery.
At its best, his play ought to be a melody of mishaps, a symphony of off-stage shenanigans, a crescendo of catastrophe. The Old Vic's clumsy revival feels instead like a game of ping-pong being played with a potato.
Sunday Telegraph, 18 Dec. 2011. The play was Michael Frayn’s Noises Off. Other reviewers were kinder about it.
Shenanigan seems to have originated in California at about the time of the Gold Rush, though it was first recorded in print only in 1855. In the years since, it has been spelt in about a dozen different ways, though recent dictionaries have settled on the version above.
Where it comes from is still a matter of substantial dispute; the first five dictionaries I consulted gave four different origins (Oxford Dictionaries — as so often — opting for the ultra-cautious “origin unknown”). The word looks Irish, and there was no shortage of Irishmen in the California diggings, so it’s plausible to suggest the Irish word sionnachuighm as the source, meaning ‘I play tricks’, which is pronounced roughly as ‘shinnuckeem’. Others argue it comes from an East Anglian dialect word nannicking for playing the fool. Yet others guess at a link with the Spanish word chanada for a trick or deceit, which is another half-way plausible source, considering California’s history. Yet another theory was put forward in 1948 in American Speech for an origin in German schinnagel for a nail that holds the rim to the wheel, which produced the German slang terms schinageln, to work, and Schenigelei, a trick.
As the man behind the market stall said, you pays your money and you takes your choice ...