It's the catchphrase that Selphie is trying to spread in FFVIII. She didn't make it very far though. I think you only see it when you access the study panel in the classroom after she starts running her website. Anyway, my question to you is this:

Have you ever said "Booyaka"?

I never did, because I really didn't know what it was supposed to mean. I think it was supposed to be a way of showing excitement, because Selphie is a really energetic and excitable character. I have other phrases that I use when I get excited over something. Also, people would probably think that I was odd if I suddenly shouted "Booyaka"!