I don’t know why, but I can’t see the word alack without the word alas following immediately behind it in my mind. There’s probably a well-known quote—it sounds like it comes from Shakespeare, but probably not—that included “alack, alas” but I don’t know.
Somebody who cares more about it than I do might want to Google “alack, alas” or maybe it was “alas, alack” to see if it is in a famous quote. Let us know in a comment here if you do.
Alack is a word you say to express sorrow. For example, I might say, “Alack, I have no more to say about the word alack.”