When a female presses her breasts up against a window, ususaly from within a car, or from a deck where a hot tub is in use. Giving the viewer on the other side of the glass a view of her pressed tits. The effect is similar to a "presses ham" where you'd see ass instead of tit.
1. A girl presses her breasts to the sliding glass deck door, from the hot tub side.
Zoe: Look! Lisa's got her tits up against the window!
Donna: Oh. My. God! She's got such great tits!
Greg: Now that's what I call some nice mashed potatoes!
Zoe and Donna, together: That's brilliant!
2. Delayed effect.
Did someone give the window a mashed potato? There's nipple prints on the glass.
3. Observed at the bar.
Her top was so tight as to give the impression that her tits were like mashed potatos.
By aspman