Derived from the word wallop, which is onomatopoeia (a sound word) for an impact of some sort, it is a Scottish slang term for a penis, usually a large penis.
It is also used to refer to unpleasant people, much like dick, cock, prick and knob would be used in conversation.
"Jesus Christ, did ye see the size of that walloper he had on him?"
A person or situation that busts your balls or an interjection expressing such ball busting, such as when you lip out a four foot put or get your shit handed to you in Mario Kart.