late 14c., alienacioun, "action of estranging, disownment;" early 15c., "transfer of one's title to property or rights," from Old French alienacion and directly from Latin alienationem (nominative alienatio) "a transfer, surrender, separation," noun of action from past-participle stem of alienare "to make another's, part with; estrange, set at variance." This is from alienus "of or belonging to another person or place," from alius "another, other, different" (from PIE root *al- (1) "beyond").
Middle English alienation also meant "deprivation of mental faculties, insanity" (early 15c.), from Latin alienare in a secondary sense "deprive of reason, drive mad;" hence alienist. Phrase alienation of affection as a U.S. legal term in divorce cases for "falling in love with someone else" dates to 1861.