A vicar of Bray; one who frequently changes his principles, always siding with the strongest party: an allusion to a vicar of Bray, in Berkshire, commemorated in a well-known ballad for the pliability of his conscience.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Brat * Brazen-facedNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes