One whose breech may be seen through his pocket-hole: this saying arose from the old philosophers, many of whom depised the vanity of dress to such a point, as often to fall into the opposite extreme.
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.
Hearty Choak * To HeaveNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes