"performer who takes the accompanying part in music," 1779, from accompany + -ist. Fowler prefers accompanyist.
Entries linking to accompanist
accompany v.
early 15c., "to be in company with," from Old French acompaignier "take as a companion" (12c., Modern French accompagner), from à "to" (see ad-) + compaignier, from compaign (see companion). The musical meaning "play or sing along with" is by 1570s. Related: Accompanied; accompanying.
-ist
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.