Somebody who's freakin' out and needs to chill. Can be used as a verb, noun or adjective.
"Yo I just ate weed
brownies and I'm hella chimped."
Person A- "Wanna hang with Brian tonight?"
Person B- "Nah, that dude's a chimp."
Person A- "Ahhh oh no (insert complaint about whatever)!
Person B- "Yooo quit chimpin'."
Person A- "Woah... you're right. I need
Person A- "Word."