Extreme comes from Latin extrēmus, the superlative of exterus meaning 'outer'. It literally means 'the most outward' — the furthest possible point from the centre.
Reaching the highest degree or furthest point; very great or intense.
From Old French extreme, from Latin extrēmus meaning 'outermost, utmost, farthest', the superlative form of exterus meaning 'outer'. The Latin exterus derives from ex meaning 'out of'. So extrēmus literally means 'the most outward'. In Latin, extrēmus was used both spatially (the farthest point) and figuratively (the utmost degree). English borrowed the word in the 15th century, initially for physical extremes — the extreme edge of a continent — before extending it to abstract intensity
Extreme is a triple superlative in disguise. Latin exterus ('outer') was already comparative in feel. Its superlative extrēmus meant 'the most outer'. When English speakers say 'most extreme', they are stacking a third layer of superlative onto a word that was already at maximum intensity two thousand years ago.