This would be used in a situation where an individual with a good behind is walking in
front of you, "shaking" their ass. By asking if they want fries with that, you're asking if they want to have
intercourse, where the
fry is meant to simbolize a dick.
Vicky: Alright, I'll see you later John.
Vicky turns around and starts walking away.
John: Would you
like some fries with that shake?