1. Something nasty, or unpleasant
2. Dark, shady, or colour drained
3. New method of saying something is cool similar to the usage of sick
2. "The weather's looking pretty grim today"
3. "You're so grim!"
It's usually a girl who loves boys and is always around them, she sucks dick and has sex and does lots of stuff with lots of different boys.
Just another way of saying a prostitue but even worse.