Offensive, chiefly Brit. Contraction of bugger (sodomise), fuck (fornicate), and twat (cunt).
Used as a superlative in the rare case that two expletives will not suffice, to describe someone so utterly contemptible and stupid that the utterer has to search her mind for a sufficiently potent approbrium.
What?!! You sold your twelve year old sister to a Romanian sex ring mafioso for half a cabbage? You... You... BUGGERFUCKTWAT!!!!!!