1690s, "pertaining to existence," from Late Latin existentialis/exsistentialis, from existentia/exsistentia (see existence). As a term in logic, "expressing or stating the fact of existence," from 1819; in philosophy, from 1937, tracing back to the Danish works of Kierkegaard (see existentialism). Related: Existentially.
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.
1941, from German Existentialismus (1919), replacing Existentialforhold (1849), ultimately from Danish writer Søren Kierkegaard (1813-1855), who wrote (1846) of Existents-Forhold "condition of existence," existentielle Pathos, etc. (see existential), and whose name means in Danish, literally, "churchyard" (compare Middle English cognate kirrkegærd (c. 1200), dialectal church-garth).
<a href="https://www.etymonline.com/word/existentialist">Etymology of existentialist by etymonline</a>
Harper, D. (n.d.). Etymology of existentialist. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/existentialist