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?
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