1. a cool/excited way of saying "yeah!" or "of course!" short for "shit yeah"
2. can also be used in a sarcastic way, as used in the movie 'Waynes World'
> 1. "You goin' to Mike's party?" "SHYEAH!... I can't wait!"
> 2. "You goin' to Mike's party?" "shyeah, right... and later on, monkies might fly outa my butt"