An attorney: those gentlemen carry their clients’ deeds in a green bag; and, it is said, when they have no deeds to carry, frequently fill them with an old pair of breeches, or any other trumpery, to give themselves the appearance of business.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Green-1 * Green GownNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes