The action of giving an article, post or video on an internet forum or website a positive rating, such as a "thumbs up".
This article is really insightful. I think I will upvote it.
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!"