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."
Someone or something that, on purpose or by accident, regularly screws up situations that take a lot of time, patience and effort to get to, including finishing an artistic masterpiece or concluding successful business negotiations.
