A blunder; from one Obadiah Bull, a blundering lawyer of London, who lived in the reign of Henery VII.: by a bull, is now always meant a blunder made by an Irishman. A bull was also the name of false hair formerly much worn by women. To look like bull beef, or as bluff as bull beef; to look fierce or surly. Town bull, a great whore-master.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Bull * Bull-2Nathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes