What does "on" mean in British slang?

In British slang, "on" is commonly used in the phrase "you're on" to mean you agree to something or accept a bet or proposal, similar to saying "deal" or "agreed." It can also appear in expressions like "what's on with you?" meaning what's wrong or what's happening with you. More notably in modern British slang, something described as "on" can mean it's acceptable, possible, or available, as in "that's not on," which means something is unacceptable or out of order. This last usage is probably the most distinctly British and widely used, expressing disapproval or that something crosses a social boundary.