This term comes from the dirty south. Most common in Atlanta, plump monkeys are fat women, usually in their 20s, that feel like when intoxicated that are no longer fat, but supermodels, and feel they are good enough to talk to good looking men. They usually hang around the bar and squeeze their fat breasts together in hopes that someone will buy them a drink, when what they really need is a Diet Coke or a swify kick in the crotch.
"Hey dogg, is that missy elliot over there?"
"nah man, its just some plump monkey that looks like her"