For a girl, a masturfail is when they finger themselves so much that eventually they made the inside of their pussy raw and in some cases, until it bleeds. For both boy and girl, it is very painful.
Guy: I would love to, but, I masturfailed last night...
Hot Chick: Oh that happened to me a few weeks ago but it's all good now. Another time then.
Guy: Yes, another time.
Guy1: Ahhh, I had this huge masturfail last night. It sucks.
Guy2: Dude! Don't tell me stuff like that!