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When one flexes his or her ass causing multiple dimples to form giving the appearance of lumps like mashed potatoes
Person 1: Oh my God, look at his ass.
Person 2: That is some seeeerious mashed potato ass.
by ABlackMan69 November 03, 2010
An older woman's ass that is rotund and full of cellulite dimples. When they sit down, the ass seems to squish and go all over the place, as if they were sitting on a pile of mashed potatoes.
My sixty-five year old teacher has a mashed potato ass. It is very distracting.
by ihatebuffering March 06, 2011