'Westernly' originates from Old English, specifically the word 'western' (from 'west' + suffix '-ern'), with the adverbial suffix '-ly' added later; 'west' meant 'west', '-ern' meant 'of or relating to', and '-ly' formed adverbs meaning 'in the manner of'.

'West' (Old English 'west') formed the adjective 'western' (Old English 'westerne'/'westren'), and English later formed the adverb by adding the suffix '-ly' (from Old English '-lic'/'-lice'), resulting in 'westernly' in modern English.

Initially, the root elements referred simply to direction or relation to the west ('of or toward the west'); over time, an additional sense developed of 'in a Western manner' (cultural/style sense), which exists alongside the directional sense today.