Roman catholics, so called from their beating their breasts in the confession of their sins. See Brisket Beater, and BREAST FLEET.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Crashing Cheats * Cream-pot LoveNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes