Furthest toward the east; most easterly (though 'easternmost' is the standard modern form).
From 'easter' (comparative 'more east') plus '-most' (superlative suffix meaning 'most'). This is an archaic or variant form of the more common 'easternmost.'
Language naturally selects efficient forms—'easternmost' (east + -ern + -most) beat out 'eastermost' because it has better phonetic flow, showing how pronunciation preferences shape which words survive.
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