A bawd. Mother abbess: the same. Mother midnight; a midwife. Mother in law’s bit; a small piece, mothers in law being supposed not apt to overload the stomachs of their husband’s children.
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.
Mot * Mother of All SaintsNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes