What is the X in X-mas?
early 15c., from Old French extreme (13c.), from Latin extremus "outermost, utmost, farthest, last," 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). The noun is first recorded 1540s, originally of the end of life, cf. Latin in extremis. Extreme unction preserves the sense of "last, latest" (15c.). Extremes "opposite ends of anything" is from 1550s.