alienyoudaoicibaDictYouDict[alien 词源字典]
alien: [14] The essential notion contained in alien is of ‘otherness’. Its ultimate source is Latin alius ‘other’ (which is related to English else). From this was formed a Latin adjective aliēnus ‘belonging to another person or place’, which passed into English via Old French alien. In Middle English an alternative version alient arose (in the same way as ancient, pageant, and tyrant came from earlier ancien, pagin, and tyran), but this died out during the 17th century.

The verb alienate ‘estrange’ or ‘transfer to another’s ownership’ entered the language in the mid 16th century, eventually replacing an earlier verb alien (source of alienable and inalienable).
=> alibi, else[alien etymology, alien origin, 英语词源]
alien (adj.)youdaoicibaDictYouDict
mid-14c., "strange, foreign," from Old French alien "alien, strange, foreign; an alien, stranger, foreigner," from Latin alienus "of or belonging to another, foreign, alien, strange," also, as a noun, "a stranger, foreigner," adjectival form of alius "(an)other" (see alias (adv.)). Meaning "not of the Earth" first recorded 1920. An alien priory (c. 1500) is one owing obedience to a mother abbey in a foreign country.
alien (n.)youdaoicibaDictYouDict
"foreigner, citizen of a foreign land," from alien (adj.). In the science fiction sense, from 1953.