1749, "pertaining to or characterized by sentiment, appealing to sentiment rather than reason," from sentiment + -al (1). At first without pejorative connotations; the meaning "too tender-hearted, apt to be swayed by sentiment" is attested by 1768 (implied in sentimentality). The French word is said to be from English. Related: Sentimentally.
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.
<a href="https://www.etymonline.com/word/sentimentalist">Etymology of sentimentalist by etymonline</a>
Harper, D. (n.d.). Etymology of sentimentalist. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/sentimentalist