The definition of alas is to express unhappiness, sorrow or regret.(interjection)
An example of alas is someone saying they wish they'd known about an event before rsvp'ing for another, such as "Alas, I wish I would have known about the event because I have already agreed to go somewhere else."
See alas in Webster's New World College Dictionary
Origin: ME < OFr a las, helas < a-, he-, ah + las, wretched < L lassus, weary
See alas in American Heritage Dictionary 4
Origin: Middle English
Origin: , from Old French a las, helas, ah (I am) miserable
Origin: , from Latin lassus, weary; see lē- in Indo-European roots.
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