can be used as an insult as well as a verb in the same way as wanker is used both as a pejorative and to describe the act of a man masturbating.
"that chick is such a masher"
if a woman cuts you off in traffic you can yell "you fucking masher"
Richard: "Who? Catherine? Squatting over the Potatoes? She must have her whole fist up there man."
Andrew: "Yeah, she's Mashing hard dude."