alienist n.

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."

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.

"one who practices psychiatry," 1875, from psychiatry + -ist.

A psychiatrist is a man who goes to the Folies Bergère and looks at the audience. [Anglican Bishop Mervyn Stockwood, 1961]

An older name was mad-doctor (1703); also psychiater "expert in mental diseases" (1852), from Greek psykhē + iatros. Also see alienist.