A diminutive person, man or woman. She was such a-hop-o-my thumb, that a pigeon, sitting on her shoulder, might pick a pea out of her a-se.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Hop Merchant * HopkinsNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes