A playing card or die showing two spots, or in slang, the number two or sometimes a best friend (from the phrase 'deuce of friends').
From Old French 'deus' and Latin 'duos' (two). The card/die meaning comes directly from counting two spots; the slang extended from there through rhyming slang and informal usage.
In tennis, 'deuce' (a variant spelling) means the score is tied at 40-40, and you need to win by 2 points—a fascinating use of the 'two' meaning that created its own special sports vocabulary, showing how games generate their own linguistic universes.
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