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.
SydneyAustralia 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.