When a man is having sex with a woman who is having her period and he pulls out and slaps his penis against her stomach or thigh, leaving a red arrow shaped mark on her skin, which resembles an upvote arrow on Reddit. Signifies a job well done, or 'thumbs up'.
"She was having her period, but we banged anyways... When we were finished I gave her an upvote, she earned it!"
by Throb_Johnson May 11, 2013