Green’s Dictionary of Slang

a long coat worn by clergymen.

1853		W.J. Conybeare Edinburgh Rev. Oct. 315: Who does not recognise .... the stiff and tie-less neckcIoth, the m.b.coat and cassock waistcoat, the cropped hair and un-whiskered cheek?
1860		Hotten Dict. of Modern Sl. etc. (2nd edn) 173: M.B. Coat i.e., Mark of the Beast, a name given to the long surtout worn by some of the clergy, ― a modern Puritan form of abuse, said to have been accidentally disclosed to a Tractarian customer by a tailor’s orders to his foreman.
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.