Terms used by printers: monks are sheets where the letters are blotted, or printed too black; friars, those letters where the ink has failed touching the type, which are therefore white or faint.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Mongrel * MonkeyNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes