More often lowercase, woo yay is used as an apathetic response to someone's irrational exuberance.
Also known as "Here's a quarter, call someone who cares."
Little Bro: OMG LOOK I GOT LEVEL 9 IN FF7!
Big Bro: woo yay.
A shout of joy. Pretty much the same as w00t!. Usually used online.
I finally beat that video game! Woo Yay!