As an exclamation of sorrow or lamentation: ‘alas!’ Also preceded by ah or O. Now archaic and regional.
Origin
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 worth the day, and also lack-a-day.