to interupt abruptly in a rude fashion, typically in the beginning of a response or sentence, commonly used as a joke.
person A: 'hey where did you purchase your t-shirt?'
person B: 'oh i got it at-'
person A: 'COO WHATEVA!!'
if a black person were to use this:
black person A: 'yo son, whats yo lil cuzin' name?'
black person B: 'aw that nigga name is-'
black person A: 'COO WHATEVA NIGGA!!'