Case

In poker, the adjective case refers to the lone remaining card, or on occasion cards, of a given rank in the deck. For example, the case deuce means the lone remaining deuce if the other three deuces are visible. If you start with two aces, the case aces are the other two in the deck.

The etymology of this term is from the game faro. Acording to this 2+2 post the reference is to an apparatus used to track cards in that game.