As an exclamation of sorrow or lamentation: ‘alas!’ Also preceded by ah or O.
Late 16th century; earliest use found in William Elderton (d. ?1592), ballad writer. Alteration of wellaway with substitution of day for the ending, after e.g. alack the day; compare e.g. woe [adverb, noun, adjective], and also lack-a-day.