"one who goes to extremes, a supporter of extreme doctrines," 1840, from extreme + -ist.
Entries linking to extremist
extreme adj.
early 15c., "outermost, farthest;" also "utter, total, in greatest degree" (opposed to moderate), from Old French extreme (13c.), from Latin extremus "outermost, utmost, farthest, last; the last part; extremity, boundary; highest or greatest degree," superlative of exterus (see exterior). In English as in Latin, not always felt as a superlative, hence more extreme, most extreme (which were condemned by Johnson). Extreme unction preserves the otherwise extinct sense of "last, latest" (15c.).
-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.