What does boo-yaa mean?

Boo- Yaa also Boo Yeah

Boo-Yaa is used to express happiness , accomplishment or an 'I told you so'

Ex- I just scored 10 points in the first round- Boo yaa!

Ex- You chose answer A but the answer was choice B, I knew it! Boo Yaa!

See the Related Link for the definition.