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An exclamation of agreement to something that was said or that is happening, often (but not always) a challenge or a wager. A: "Want to see who can run to the end of the road the fastest?" B: "You're on!" A: "I bet you $50 that my team beats yours in the championship." B: "You're on!"
See also: on
1 used for showing that you agree to somebody’s good, interesting, etc. suggestion: ‘You pay for the wine and I’ll pay for the food.’ ‘You’re on!’
2 used for accepting a bet from somebody: ‘I bet you €10 that Manchester United beat Barcelona.’ ‘You’re on!’