When a guy is giving a girl oral sex and shakes his face back and forth in her vagina
"She got pretty pissed when I started fish slapping her"
To slap someone with a fish. Often used in nonsense shows, movies and stories.
Cuzzin Red- "Stawp fish slappin' those inocent people! They never dun anything ta y'all!"
The Masked FIsh SLapper- "I will go around, fish slapping whoever I want!"