whilst having sex with ones girlfriend, wife, or boyfriend i suppose, the screen in a window is removed, and the receiver is hung out the window. the giver then repeatedly slams the window frame down on the penis, resulting in pain and satisfaction at the same time.
Dude 1: hey man, i totally windowpained with megan last night!
Dude 2: wicked! how was it?
Dude 1: o it hurt so good!
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.