A close relative of an asshat, an assmonkey is best defined as 'any disagreeable idiot who has successfully combined being an asshole with the brainpower of a monkey fed LSD'.
Eytemology is unknown, but there are perhaps links to Wayne's World, and the line "Pshyeah, and monkeys might fly out my butt."
Similar to the idiom, "Wear Your Heart On Your Sleeve" but instead of expressing your emotions freely, you (or someone else) show how much of an asshole you are.
Cal: Yeah, so I punched him in the face for taking the last slice of pizza.
Rudi: It was his birthday, assjacket.