The term developed because during fellatio, it seems that the person performing oral sex is eating around the zipper of their partner’s pants.
The term zipper dinner is commonly believed to originate in the United States.
More About Zipper Dinner
While zipper dinner is typically used to describe any type of fellatio, some suggest that it may satisfy certain criteria to be called a zipper dinner.
Some believe the fellatio must be performed to orgasm. Others suggest a zipper dinner should be used exclusively for homosexual fellatio where the person being fellated keeps his pants on, usually because the act occurs spontaneously in a moment of passion.
There is also an assumption that the person fellating does so enthusiastically, as if they are devouring a good meal.