you're on

you're on

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
Farlex Dictionary of Idioms.

you're on

said by way of accepting a challenge or bet. informal
See also: on
Farlex Partner Idioms Dictionary

you’re ˈon!

(informal)
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!’
Farlex Partner Idioms Dictionary
See also:
  • hoo-rah
  • viola
  • voilà
  • ah
  • Ding, ding, ding, we have a winner!
  • gotcha
  • be still my beating heart
  • be still my heart
  • boom goes the dynamite
  • dynamite