c. 1300, "strange, foreign," from Old French alien "strange, foreign;" as a noun, "an alien, stranger, foreigner," from Latin alienus "of or belonging to another, not one's own, foreign, strange," also, as a noun, "a stranger, foreigner," adjective from alius (adv.) "another, other, different" (from PIE root *al- (1) "beyond").
The meaning "residing in a country not of one's birth" is from mid-15c. The sense of "wholly different in nature" is from 1670s. The meaning "not of this Earth" is recorded by 1920. An alien priory (mid 15c.) is one owing obedience to a religious jurisdiction in a foreign country.
word-forming element in nouns of act, process, function, condition, from Old French and French -age, from Late Latin -aticum "belonging to, related to," originally neuter adjectival suffix, from PIE *-at- (source of Latin -atus, past participle suffix of verbs of the first conjugation) + *-(i)ko-, secondary suffix forming adjectives (see -ic).
<a href="https://www.etymonline.com/word/alienage">Etymology of alienage by etymonline</a>
Harper, D. (n.d.). Etymology of alienage. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/alienage