Green’s Dictionary of Slang

a kind of waistcoat with no opening in front, worn by Anglican clergymen.

1851		Macaulay letter 24 Mar. in Pinney (ed.) Letters V (1974-81) 155-6: ‘What does an M. B. waistcoat mean?’ ‘Oh, Sir, all the trade know it by that name.’ ‘But what does it mean?’ ‘The Mark of the Beast, Sir.’.
1861		Hills & Plains I 135: M. B. waistcoats, parched-pea dietaries on fast days, altar flowers and lights, gorgeous vestments, and such other bones of contention between High and Low.
1866		Letters by an Odd Boy 161: I turn to a gentlemanly friend of mine, who holds a living pro tempore, and am taught to call him a ‘warming-pan rector,’ ‘a howling swell; ‘his black silk waistcoat of the ‘M.B.’ (mark of the beast) pattern, and his neckerchief ‘a choker!’.
1884		Graphic 20 Sept. 307/2: He has begun to affect the strictest clerical garb – m.b. waistcoat, hard felt hat with band and tassels.
1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.