Used to indicate a burn or a diss. Made popular by that episode on Scrubs where Dr. Cox hates on the Indian guy because he's lazy.
Dr. Cox: Who made that noise?
Ed: Oh, uhh... I did. I'm a little tired of her chipper attitude so it's cool to see you just slam her so I just went Zwaa!
Dr. Cox: So, if I were to ask you a question like "What is the next diagnostic move in checking for Zollinger–Ellison?" and you didn't know I would say "Zwaa!"
Ed: Nope. Because I know the answer: Esophagogastroduodenoscopy. Zwaa!
Dr. Cox: Good. But mark my words I will Zwaa you by the end of the day.