Wikipedia
Up Jenkins, also known by the shortened name Jenkins, is a party game in which players conceal a small coin (or ring, button, etc.) in their palm as they slap it on a table with their bare hands. The goal of the game is for the players on the team without the coin to correctly identify which hand the coin is under. The game typically consists of two two- to four-player teams, one on each side of a table. There are no official rules, so rules may vary widely, the game is often played with alcohol beverages with which to drink as a forfeit.
Usage examples of "up jenkins".
It was the first time he'd seen any real enthusiasm on the boy's face, and however good or bad the nurse was, she'd obviously be of value in bucking up Jenkins' spirits.
I'll go outside and wake up Jenkins and have him get your truck ready.