An alternative name for a blowjob, or to suck someone off.
This word came about as, when performing oral sex, you neither "blow", or "suck".
It's just a bit of a mouthwank really.
Girl: I enjoyed the movie, and I'm not tired at all!
Boy: Well baby, I'm up for a mouthwank.
Girl: Yaaaaay!
Boy next day....
Boy: So I was wiv my bitch Chardonnay, and she totally mouthwanked me!
Friends: Dat is well gud