This is generally said by people who are afraid of saying the 's' word as they have spent the first four decades of their pitiful lives stuck to their mothers nips.
Ted: "Did you see that new adult site?"
Phil: "Yeah, I got some great rumpy bumpy viedos of it."
rump bump means that when a man grabs a woman/man by his/her hips and shoves his hard erect penis into her ass crack, while in the process of pulling his/her pants down. then, they shove the person on the ground, get on top of the person and rides them like a pony, then you go 69 on their ass
damn bitch, i had sonia over last night and i tried to rump bump her ass but it was too flat and nasty, and she was terrible.
damn hoe, i had m&b in bed last night, they were the best rump bumpers i know.