What to call sex between two old people (30+) as the man is past his peak (rotten potatoes) and the woman is expired (rotten milk).
"Ew, I caught my grandparents doing the raunchy mashed potatoes!"
When you're unaware of your pregnancy and you still have sex.
"What the hell? I don't what happened to Jimmy, but he be looking like he got the mashed potato. Not sure what happened to him, but when he came out, he came out in pieces."