deuce: [15] The underlying meaning of deuce emerges most clearly in its application to playing cards and dice: the number two. It comes from Old French deus ‘two’, which goes back to duōs, the accusative case of Latin duo ‘two’ (English two comes ultimately from the same source). Its use in tennis comes from the French phrase à deux, literally ‘a(chǎn)t two’, which signifies that a player must score two successive points to win a game. It is generally thought that the mild oath deuce came from duus, the Low German descendant of Latin duōs, which gamblers supposedly cried out in disgust when they threw the lowest score, a two.
deuce (n.)
late 15c., "the 2 in dice or cards," also "a roll of 2 in dice" (1510s), from Middle French deus (Modern French deux), from Latin duos (nominative duo) "two" (see two).
Became a mild oath by 1710, about 50 years after it was first attested in the sense of "bad luck, the devil, etc.," perhaps because two was the lowest score, and probably by similarity to Latin deus and related words meaning "god." Low German had der daus! in same sense 16c., which perhaps influenced the English form. Deuce coupe is 1940s hot-rodder slang for "souped up two-door car," especially a 1932 Ford. Related: Deuced; deucedly.