Deuce (dūs), n. [F. deux two, OF. deus, fr. L. duo. See Two.] 1. (Gaming) Two; a card or a die with two spots; as, the deuce of hearts.
[1913 Webster]

2. (Tennis) A condition of the score beginning whenever each side has won three strokes in the same game (also reckoned “40 all”), and reverted to as often as a tie is made until one of the sides secures two successive strokes following a tie or deuce, which decides the game.
[1913 Webster]