a sovereign; Half a Quid, half a sovereign; Quids, cash or money generally. A suggested derivation may be mentioned. Quo = anything, and Quid pro quo means an equivalent generally. If now a person is offered anything on sale he might say, I have not a quid for your quo, an equivalent in cash.
“Then, looking at the gold piece, she added, `I guess you don't often get one of these quids.' ” —Liberty Review, June 9, 1894, p. 437.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Quid from Infoplease: