1590s, "pledge as a forfeit to another who makes a similar pledge in return," originally in the argot of petty criminals, a word of unknown origin; see bet (n.), which appeared about the same time. Intransitive sense of "lay a wager" is from c. 1600. Used since mid-19c. in various American English slang assertions (bet your life, 1848; bet your boots, 1856; you bet "be assured," 1857, identified in Century Dictionary as "originally California slang").
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Definitions of betting from WordNet
preoccupied with the pursuit of pleasure and especially games of chance;