psychologist (n.)

1650s, "the study of the soul," from Modern Latin psychologia, probably coined mid-16c. in Germany by Melanchthon from Latinized form of Greek psykhē "breath, spirit, soul" (see psyche) + logia "study of" (see -logy). The meaning "science or study of the phenomena of the mind" is attested by 1748, in reference to Christian Wolff's "Psychologia empirica" (1732). The modern behavioral sciences sense is from the early 1890s.

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.

1925 as short for psychologist; (earlier short for psychology, 1921); as short for psychopath by 1942.