The verb 'To Waz' in northern English dialect means to urinate.
In both West Yorkshire and East Lancashire, offal was very popular and when produced en masse the offal was cooked in linens resembling large pillows and were referred to as 'socks'. These prevented the offal from burning and being ruined when coming in to direct contact with the base or sides of its cooking vessel.
Unsurprisingly these linens absorbed the odour of the offal (which is similar to urine) and were discarded. They were colloquially referred to as 'Waz Socks'. The derogatory sense of a 'Wazzock' to this day refers to someone discarded of having any point, purpose or worth.
Nobody knows where or when this word originated but several plausible explanations date back to Yorkshire and the 1960's including that it is a corruption of the french "oiseau" meaning "bird" as used in the phrase ouiseau head.
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.
Popularised by Tony Capstick (in "Capstick Comes Home") in 1981 and used since as a minor insult (similar to how "idiot" is used as an insult).
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.
You great useless spawny-eyed parrot-faced wazzock
(Tony Capstick, 1981)