E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A name given, to which guests are invited to drink in compliment. The name at one time was that of a lady. The word is taken from the toast which used at one time to be put into the tankard, and which still floats in the loving-cup, and also the cups called copus, bishop, and cardinal, at the Universities. Hence the lady named was the toast or savour of the winethat which gave the draught piquancy and merit. The story goes that a certain beau, in the reign of Charles II., being at Bath, pledged a noted beauty in a glass of water taken from her bath; whereupon another roysterer cried out he would have nothing to do with the liquor, but would have the toasti.e. the lady herself. (Rambler, No. 24.)
Let the toast pass, drink to the lass.Sheridan: School for Scandal.