'westernised' originates from Modern English, specifically from the adjective 'western' combined with the verb-forming suffix '-ise' (British) or its variant '-ize' (American); 'western' itself comes from Old English 'western', ultimately from Proto-Germanic and Proto-Indo-European roots where the stem meant 'west' (the direction of sunset).

'western' existed in Old English as 'western' (meaning 'of the west'); the verb-forming suffix entered English via Old French and Latin/Greek ('-iser' / '-izare' / '-izein'), producing 'westernise' in modern English; the past/past-participle form subsequently became 'westernised' in British spelling (and 'westernized' in American spelling).

Initially it described something 'of or from the west' (adjective). Over time a verbal sense developed meaning 'to make or become Western in character or practice,' which is the primary sense of 'westernise' and thus of 'westernised'.