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."
Meaning "residing in a country not of one's birth" is from mid-15c. Sense of "wholly different in nature" is from 1670s. Meaning "not of this Earth" first recorded 1920. An alien priory (mid 15c.) is one owing obedience to a religious jurisdiction in a foreign country.
Proto-Indo-European root meaning "beyond."
It forms all or part of: adulteration; adultery; alias; alibi; alien; alienate; alienation; allegory; allele; allergy; allo-; allopathy; allotropy; Alsace; alter; altercation; alternate; alternative; altruism; eldritch; else; hidalgo; inter alia; other; outrage; outrageous; outre; parallax; parallel; subaltern; synallagmatic; ulterior; ultimate; ultra-.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit anya "other, different," arana- "foreign;" Avestan anya-, Armenian ail "another;" Greek allos "other, different, strange;" Latin alius "another, other, different," alter "the other (of two)," ultra "beyond, on the other side;" Gothic aljis "other," Old English elles "otherwise, else," German ander "other."
1510s, "transfer to the ownership of another;" 1540s, "make estranged" (in feelings or affections), from Latin alienatus, past participle of alienare "to make another's, part with; estrange, set at variance," from alienus "of or belonging to another person or place," from alius "another, other, different," from PIE root *al- (1) "beyond." Related: Alienated; alienating.
In Middle English the verb was simply alien, from Old French aliener and directly from Latin alienare. It is attested from mid-14c. in theology, "estrange" (from God, etc.; in past participle aliened); late 14c. as "break away (from), desert;" c. 1400 in law, "transfer or surrender one's title to property or rights."
"act of naturalizing; state of being naturalized;" specifically in reference to the act of receiving an alien into the condition of a natural citizen, 1570s, from French naturalisation, noun of action from naturaliser (see naturalize).