'Easternly' originates from Old English, specifically the word 'ēast', where 'ēast' meant 'east', combined with the adjectival suffix '-ern' (Old English) and the adverbial suffix '-ly' (from Old English '-lic' > Middle English '-ly').

'Easternly' changed from Middle English 'esternly' (from 'estern'/'eastern') and eventually became the modern English word 'easternly'.

Initially, it meant 'of or toward the east', and over time it has retained much the same meaning, though modern usage often prefers 'easterly' or 'eastern'.