psychiatrist (n.)

"the medical treatment of mental diseases," 1846, from French psychiatrie, from Medieval Latin psychiatria, literally "a healing of the soul," from Latinized form of Greek psykhē "mind" (see psyche) + iatreia "healing, care" (see -iatric).

word-forming element meaning "one who does or makes," also used to indicate adherence to a certain doctrine or custom, from French -iste and directly from Latin -ista (source also of Spanish, Portuguese, Italian -ista), from Greek agent-noun ending -istes, which is from -is-, ending of the stem of verbs in -izein, + agential suffix -tes.

Variant -ister (as in chorister, barrister) is from Old French -istre, on false analogy of ministre. Variant -ista is from Spanish, popularized in American English 1970s by names of Latin-American revolutionary movements.

"one who scientifically treats or studies mental illness," 1864, from French aliéniste, from alienation in the sense of "insanity, loss of mental faculty," from Latin alienare "deprive of reason, drive mad," literally "to make another's, estrange" (see alienate). The mental sense of alienate has since mostly died out in English, but Middle English had aliened from mind "deranged, not rational" (late 14c.), and alienation was used from 15c. in a sense of "loss or derangement of mental faculties, insanity."