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?"
Sydney Australia slang for a police officer. Based upon the belief that police are inherently violent, and therefore if they speak to you then you are likely to be walloped (beaten, assaulted).
I saw the wallopers patrolling past my house but they didn't stop, so someone else must be in line for a beating tonight.