It means to clap them cheeks, get some ass. Used as a metaphor for Butt cheeks since they resemble dunes, thus you beat the Dunnies Down.
Yo I took your mom home last night and beat her Dunnies Down.
Meaning is hittin it from behind and clappin them cheeks. Gets its name for resembling sand dunes, hence you beat the dunnies down.
Splez: Ayo treesh lemme beat the dunnies down!!!
Treesh: Okay daddy you can get all the ass you want...
Splez: Ight bet. Uh Huhh Huhhh
89π 4π
beat the dunnies down
Stranger : man Iβll love to beat her dunnies down respectfully
Stranger 2 : what do you mean?
Stranger 1 : you know what I meannnn
Both strangers : oooooohhh
βbeat the dunnies downβ
17π 3π
meaning to have sex with someone
let me beat the dunnies down
13π 1π
βAyo treesh lemme beat em dunnies downβ
βYou can beat emβ
6π 1π