Synonymous with male ejaculate, cum, "jizz" or a "nut". Refers to the ejaculate fluid as a whole, not just the sperm. It is a particularly good word to use when you are trying to describe a situation in very disgustingly detailed fashion.
Dude 1: Howd it go with that slut last night?
Dude2: Bagged her bro! She totally sucked the man mess right out of my jizz tube!
Dude 1: So I was beating of yesterday and blew my man mess all over my keyboard!
Dude 2: I don't want to hear that shit!
Semen; Sperm; Ejaculation
I cannot wait to spread my man mess all over her face.