BARGAIN. To sell a bargain: a species of wit, much in vogue about the latter end of the reign of Queen Anne, and frequently alluded to by Dean Swift, who says the maids of honour often amused themselves with it. It consisted in the seller naming his or her hinder parts, in answer to the question, What? which the buyer was artfully led to ask. As a specimen, take the following instance: A lady would come into a room full of company, apparently in a fright, crying out, It is white, and follows me! On any of the company asking, What? she sold the bargain, by saying, My a-se.
-- From Grose's classical dictionary of the vulgar tongue--