Defined as 'a general term of contempt for saucy or paltry fellows', 'Jack' also evokes Latinate terms like jactator ('a cracker or boaster') and jactancy ('a vain boasting').
(23) See Samuel Johnson, Dictionary of the English Language (1755; facs, Tokyo, 1983), sv 'Jack'; Thomas Blount, Glossographia; or a Dictionary (1661), EEBO, sv 'Jactator'; and Elisha Coles, An English Dictionary (1677), EEBO, sv 'Jactancy'.