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?