'westmost' originates from Old English, specifically the word 'westmǣst', where 'west' meant 'west' and '-mǣst' was a superlative suffix meaning 'most'.

'westmǣst' transformed into the Middle English word 'westmest', and eventually became the modern English word 'westmost'.

Initially, it meant 'most west', and this meaning has largely remained the same in modern usage.