E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Cheap Jack.
Jack, the chap-man. Not cheap, meaning low-priced, but cheap meaning merchant, as in “chapman,” “Cheap-side,” etc. Jack is a term applied to inferior persons, etc. (Saxon, cepa, a merchant; ceapian, to buy; ceapmann, a tradesman.) (See JACK.)
