Quick definitions from WordNet (unmodifiable)
▸ adjective: incapable of being modified in form or character or strength (especially by making less extreme) ("These variations from custom are illogical, incomprehensible, and unmodifiable")
▸ adjective: incapable of being modified in form or character or strength (especially by making less extreme) ("These variations from custom are illogical, incomprehensible, and unmodifiable")