alienist (n.)

"one who scientifically treats or studies mental illness," 1864, from French aliéniste, from alienation in the sense of "insanity, loss of mental faculty" (compare alienation, since 15c. as "loss or derangement of mental faculties, insanity"), from Latin alienare "deprive of reason, drive mad," literally "to make another's, estrange" (see alienate).