Probably derived from Old Irish
ingen meaning "daughter" or "girl" (see
Imogen).
This was the form of
Ignoge used by Milton. (According to Geoffrey of Monmouth, Ignoge was the name of a princess who was given in marriage by her father, King Pandrasus of Greece, to the Trojan exile Brutus in exchange for Pandrasus' freedom. In Britain, she became the mother of Locrine, Albanact and Humber.)
The spelling
Inogen was used by Richard Hole in his prose
Arthur (1789), where the name belonged to the daughter of Merlin, later the wife of Arthur.