Definition
War, one of the Four Horsemen of the Apocalypse.
Origins
A reference to the Four Horsemen of the Apocalypse, as the rider of a red horse is War.
How People Actually Use It
When a guy is laying on his back and a chick with a heavy period is riding on top of him, and a bit of the menstrual blood leaks out from her pussy all over his stomach and sheets.
"Ric: Yeah, I took this bitchin' ass chick home last night but she didn't tell me she was raggin. She was fuckin me on top and the next thing I know... Hamilton:... oh no, she didn't... Ric: Yep, fucking red rider all over my sheets! Bitch!!!"
Source: Urban Dictionary