When an older woman with sagging breasts forces them together in an under-sized bra to gain attain the illusion of perkiness or false cleavage. The result is a singular mound in the center of her chest while still being able to see the ill fitting bra.
Phil: Aww man. Look at that old lady's mounder.
Sam: Oh no. Well u can't motor boat that. But you can flounder a mounder.
A deep rooted zit that doesn't quite come to the surface. Often is correlated with a throbbing pain.
The Mounder on my butt prevented me from sitting down.