A poor female relation, and humble companion, or reduced gentlewoman, in a great family, the standing butt, on whom all kinds of practical jokes are played off, and all ill humours vented. This appellation is derived from a mountebank’s servant, on whom all experiments used to be made in public by the doctor, his master; among which was the eating of toads, formerly supposed poisonous. Swallowing toads is here figuratively meant for swallowing or putting up with insults, as disagreeable to a person of feeling as toads to the stomach.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Tizzy * ToadNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes