An awkward neighbor or boyfriend/girlfriend in your dorm who invites himself/herself to meals, parties, and other social events. Ass monkeys attach themselves to specific groups or individuals by seeing that other individuals are able to invite themselves to events but still be welcomed companions.
Sketchball A: We need to go out the side door. If we don't your ass monkey will come with us to dinner.
A person who, for all intents and purposes, has never learned a god-damned thing. One who maintains a blissful ignorance of the world that simultaneously inspires amusement, disgust and despair in all who are unlucky enough to encounter him.