foreignyoudaoicibaDictYouDict[foreign 词源字典]
foreign: [13] Etymologically, foreign means ‘out of doors’. It comes via Old French forein from Vulgar Latin *forānus, a derivative of Latin forās ‘out of doors, abroad’. This originated as the accusative plural of *fora, an unrecorded variant form of forēs ‘door’ (to which English door is related). The literal sense ‘outdoor’ survived into Middle English (the chambre forene mentioned by Robert of Gloucester in his Chronicle 1297, for instance, was an ‘outside loo’), but by the early 15th century the metaphorical ‘of other countries, abroad’ had more or less elbowed it aside.
=> door, forest, forfeit[foreign etymology, foreign origin, 英语词源]
foreign (adj.)youdaoicibaDictYouDict
c. 1300, ferren, foran, foreyne, in reference to places, "outside the boundaries of a country;" of persons, "born in another country," from Old French forain "strange, foreign; outer, external, outdoor; remote, out-of-the-way" (12c.), from Medieval Latin foraneus "on the outside, exterior," from Latin foris (adv.) "outside," literally "out of doors," related to foris "a door," from PIE *dhwor-ans-, from root *dhwer- "door, doorway" (see door).

English spelling altered 17c., perhaps by influence of reign, sovereign. Sense of "alien to one's nature, not connected with, extraneous" attested late 14c. Meaning "pertaining to another country" (as in foreign policy) is from 1610s. Replaced native fremd. Related: Foreignness.