sell a bargain

Wiktionary

1. v. idiomatic, obsolete, slang 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 frightened, crying out "It is white, and follows me!" As soon as someone responded "What?" she sold him the bargain, by saying "Mine arse".