You won't

An expression used to challenge a person to commit an action.

Appropriate responses are as follows:
In agreement to do the action = "Who Won't?"
Not in agreement to do the action = "You right"
Absolutely not in agreement to do the action = "Are you dumb?"
To counter challenge = "YOU won't!"
When used in an overly serious manor "you won't" may be interchangeable with "You will not!"
Warning: Increased use of alcohol can and will cause excessive repetition.

Inverse psychology that works with many but not with everyone.

The act of saying the phrase "YOU WON'T!" after someone announces they're going to do something. Generally used to be funny, and/or taunt the individual.

Whenever someone says they'll do something, someone else comes back with "You won't!" It's almost like a dare. If the person does said thing, the "You won't"-er usually comes back with something like "OK, so maybe you will."
Commonly used by the surf-bum kids who pioneered such gems of slang like "Dude" and "Sweet".

I phrase used by Jake Paul and Team 10. Only dumb people say this phrase.

a phrase to be uttered when doubt that an individual will follow through with an action is present.

expression used by people, often while playing sports in gym or practicing for their band; challenge of skill;