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.).
1540s, "utmost point of a thing," from extreme (adj.); originally of the end of life (compare Latin in extremis in reference to the "last stages of life"). Phrase in the extreme "in an extreme degree" attested from c. 1600. Hence extremes "extremities, opposite ends of anything" (1550s); also "extreme measures" (1709).