The equivalence to a homo- sexual man, (faggot, fag, fudgepacker,cock-rocker etc). Originated in the late 1970's, early 80's New York City. Gay men were spotted jerking each other off, and if seen from a distance, or from behind the recipient, it appears as though the jerker is "punching his pants," hence pants puncher.
"While driving by the YMCA, I saw a man on his knees, punching another man in the crotch repeatedly. I soon realized that I was mistaken, for the recipient of the punches was not wincing in pain, and as I drove closer I could see that the "pants puncher" was actually jerking off the standing man."
by Robert McMahon April 10, 2007