A made citizen —i.e. an alien who has been naturalised by letters patent. (Old French deinzein; Latin de-intus, from within.)
“A denizen is a kind of middle state, between an alien and a natural-born subject, and partakes of both.” —Blackstone: Commentaries, book i. chap. x. p. 374.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894