Shenanigans are silly or mischievous behaviors or activities. A shenanigan is a trick, usually of a questionable nature. Depending on the dictionary, the noun is listed as plural or in its singular form. It is used either way. It is pronounced \shə-ˈna-ni-gən\ (shi nan i gan) with both the e and the i producing the short i sound.
It was first used in the middle of the 19th century in California during the Gold Rush, but we don’t actually know where it came from before that, which seems appropriate.
You will see from the ngram below that shenanigans is more prominent than shenanigan. Also, the term may have been coined in the 1800s, but it has come into its stride in the 20th and 21st centuries.