When a guy is doing a girl and pulls out, then jizzes onto a piece of paper from a novel, preferably the
book "tai pan" by
james clavell, then proceeds to smack the piece of paper on the
girls face, imitating the act of putting a postage stamp on a letter.
Matt: I totally postage stamped this girl the other day.
Heidi: that's really sweet
OR, used in the present tense
Matt: shut up
bitch or i'll postage stamp you