When semen gets mixed up with the snot in your
nose.
Most typically occurs when deepthroating, and the
ejaculation is akin to a
rocket explosion - shooting the semen up the throat as opposed to being swallowed.
Girl: "
Babe, I think a little bit of cum came out of my nose when I blew my nose this
morning."
Boy: "Oh yeah, that's snemen from when I blew my load up your throat last
night."