“In your face” is a recent American idiom. It can have two meanings. The first, and most common, is a form of derision. The second means someone or something is brash, arrogant, or extreme. It also has an even more recent abbreviated off-shoot, “face” or “your face.”
Example of the first: “Ha! I scored! In your face!”
Example of the second: “That new band was in-your-face loud!”
Example of the latter: “I think you’re stupid,” Sue said. “Your face is stupid,” replied Dave.
Where did it come from?
‘Stuffed!’ shouted the taller boy. ‘Doobie got himself stuffed!… In yo’ face, Doobie!’
It was originally associated with sports and competition but has since evolved to a broader use for any kind of confrontation and usually used with humor.