1580s, "an account of daily events, a journal kept by one person of his or her experiences and observations," from Latin diarium "daily allowance," later "a journal," neuter of diarius "daily," from dies "day" (from PIE root *dyeu- "to shine," in derivatives "sky, heaven, god"); also see -ary.
Sense of "a book with blank leaves or dated pages meant for keeping a daily record of events" is from c. 1600. Related: Diarial; diarian.
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/diarist">Etymology of diarist by etymonline</a>
Harper, D. (n.d.). Etymology of diarist. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/diarist