a phrase used to tell somebody they're crazy after making an absurd comment.
basically it's something used to say "you must be out of your mind," or "you must be crazy."
"Demarcus, I know you gunna let me copy your homework"
Commonly used to tell someone that the statement they just made is completely insane.
Girlfriend: Baby, will you buy me this $50 dollar dress?
Boyfriend: Kill yo'self!