Originated on the site ChavScum.co.uk, used to describe the odd relationship allegedly inbred chavs have with other (male) members of their family.
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.