A health; also a beautiful woman whose health is often drank by men. The origin of this term (as it is said) was this: a beautiful lady bathing in a cold bath, one of her admirers out of gallantry drank some of the water: whereupon another of her lovers observed, he never drank in the morning, but he would kiss the toast, and immediately saluted the lady.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Toad * Toasting IronNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes