A term used in reply to someone who is making a statement that you either disagree with or say just for the hell of not agreeing with them.
Most often used when someone does not want to admit something.
*The only way something can be proven is with a pencil sketch showing that the statement is in fact valid.
Dave: Dude, Wendy is so hot!
Thomas: Prove it.
Dave: I have a picture, look!
Thomas: NO! That isn't a pencil sketch.
Chris: I made some awesome cupcakes yesterday.
Andy: Prove it.
*Chris draws a picture of himself making cupcakes entitled, Yesterday*
Chris: There! I proved it!
Andy: True dat!