Cutting the braces of coaches behind, on which the coachman quitting the box, an accomplice robs the boot; also, formerly, cutting the back of the coach to steal the fine large wigs then worn.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Chivey * ChoakNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes