My favourite is that it was used on a local football field when a particularly disenchanted supporter shouted that the referee was a "wazzock" it being a portmanteau word made up from "wanker" and "pillock" - but surely that would be a "wannock" "wallcok" (which doesn't work).
Mike Harding (ex-comedian, now radio 2 presenter)says he used the word as early as 1976 in his surreal story, "Beaky Knucklewart",recorded on his "One Man Show" album and also in "When the Martians Landed in Huddersfield" but Harding says he heard it from others probably in Barnsley in the late 1960's. This pre-dates Tony Capstick by 5 years.
Originally meant "bull's penis" (originally described to me as "bull's prick"). See also "wazz" for another derivative of this sense of the word. Whether it was ever applied to other animals or humans I don't know - but as "wazz" is used for human urination in the West and North of England, I'd imagine so.
Was fairly rare in Yorkshire before Capstick's popularisation of it in 1981.
(Tony Capstick, 1981)