going mashed potatoes
Going "Mashed Potatoes" on someone is where you perform the act of pulling down your partner's pants, smearing mashed potatoes (any flavor will do) onto their buttocks, and "motorboating" them between the cheeks. If done correctly there will be mashed potatoes strewn about the area where the act had taken place.
Tom: "Check out that girl in the yoga pants", Earl: "Oh yeah, if I had the chance I'd be going mashed potatoes on her all day"