Definition: (Colloquial) A playful or mischievous act; a prank; a secret scheme or machination.
Usage: Today's word is more usually used in the plural: "Keep an eye on Elizabeth Crowley; she is always up to some kind of shenanigans." It is an orphaned oddity in the language with no other words that would admit to kinship.
Suggested Usage: This is not a word used in formal English but is common in colloquial English. Shenanigans are usually something people are up to, "Will we ever know what sort of shenanigans the financial officers of Enron were really up to?" When anything of value is at stake, shenanigans are something you would want to cut out: "Cut out the shenanigans with the cards up your sleeve, Dizzy Lizzy, and let's play poker."
Etymology: Although the word sounds a lot like "she-nanny-goat" and even though goats are associated with prankishness, there is no evidence of any association between these words and today's. "Shenanigan" was first recorded in 1855 in California, which has led to speculation that it is related to Spanish chanada "trick" or the German schinaglen "to trick." In all probability, though, it was another word brought over by our Irish brethren: Gaelic sionnachuighim "I play the fox," which is to say, "I play tricks." –Dr. Language, YourDictionary.com