a joker in the deck

Someone or something that is unpredictable; a wild card. There are a few unpredictable elements here, but the weather will be a real joker in the deck. The new quarterback has proven to be a joker in the deck for this team. They're looking for a steady, reliable candidate for the position, not a joker in the deck.

joker in the deck, a

Any item or device, such as a clause in a contract, that unexpectedly changes the nature of something, either for better or for worse. The noun “joker” alone has been used in this way since the mid-1800s, as in “The postal rate increase had within it a joker that would hurt nonprofit organizations.” Today the term, also put as joker in the pack, sometimes refers to a person whose behavior is unexpected on unpredictable.