"let's grim up and finish this project."
"hahahaha, good joke. but let's get grim and do this."
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.
"The desolate crater that was at some time too far back to remember, a thriving populace, only produced a feeling of dread, as he looked upon it's grim countenance".