'eastmost' originates from Old English, specifically the word 'ēastmōst', where 'ēast' meant 'east' and 'mōst' meant 'most'.

'ēastmōst' changed from Old English word 'ēastmōst' and eventually became the modern English word 'eastmost'.

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