The definition of alack is a word used to express sorrow or regret.
An example of a time that someone might use the word alack is when he is expressing regret for letting the girl of his dreams get away.
Archaic used to express regret, surprise, dismay, etc.
Origin of alacka(h) + lack
Used to express sorrow, regret, or alarm: “Whatever I long for makes me cry alack. / Whatever I wish, I quickly take it back” (Richard Wilbur).
Origin of alackOn the model of alas; see lack.