The story
Possibly from wiseacre, influenced by pillock, or possibly wazz + -ock. First attested in the 1984 novel When the Martians Land in Huddersfield by Mike Harding. Harding (born 1944) later claimed that he "used it when I was a kid,” it being “fairly common in the north, in Lancashire and Yorkshire”, and suggested that the term came from the instrument used to fling a monarch's excrement out the window.
Etymology from Wiktionary, CC BY-SA 4.0.