Banging the bongos is when a sexual partner spanks a partner's butt cheeks during anal sex or while in the doggie style position.
More About Banging the Bongos
Bongos are an Afro-Cuban percussion instrument that are played by hand. Banging the bongos is sex slang meant to represent the similar action of hitting a drum by spanking your partner during anal sex or while in the doggie style position.