Said after someone says something that one goes against or doesn't agree with; something irrelevant or completely unbelievable. Kill it (definition #7) may also be added as a way to get the point across clearer.
Kathy: Britney Spears is so hot right now.
La' Sha: Girl, get down!
Lauren: Ludacris is so much finer than Usher.
Karee: Girl, get down and kill it!