Whilst in the act of intercourse, the man picks the woman up and stuffs her through an open window; while she supports herself in the window, the man aggressively slams the window shut repeatedly on his penis, which is resting in the windowsill.
Can also be used as a verb, windowpaining.
dude1: Dude, did you see that girl hanging out of that window naked? I wonder what she was doing.
dude2: It's just the old windowpain my friend.
dude1: oh man dude, my dick hurts so bad right now. I windowpained the shit out of Kelsey last night.
dude2: That's so hot, man, I wish Amanda would let me do it to her.