Originated on a certain anti-chav site, used to describe the odd relationship apparently inbred chavs have with other (male) members of their family. (SEE Jug Ears
. ALL chavs have jug-ears.)
Probably obviously, it derives from brother, uncle, and dad, and it has been quoted as "merely being uncle dad with 2 extra letters".
No-one, as of yet, has managed to figure out how a person might become brother, uncle, and father all to the same person.
A suitably humourous female counterpart has yet to be coined.
That chavette only has a dummy in her mouth to keep in all the spooge she drained from her bruncledad's nutsack.