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?"
or
"He's a bleedin' walloper, and that's all there is to it!'
person of or from Walton, an area of Liverpool so backward that most L9 dwellers claim to hail from neighbouring Aintree; often a practicioner of the renowned 'Walton Bird' gesture.