In this context Muffin means Pussy or Vagina. The Masher is another female who rubs their Pussy " muffin" against the afore mentioned Muffin, for sexual purposes. In most cases a Muffin Masher is a bull dike but in some cases it could be just a one time experimentation thing for a female.
1. Holy shit Jill turned to Muffin Mashing!
2. Jill say's " I've always wanted to try Muffin Mashing."