Alas is another way to emphatically say “unfortunately.” Perhaps you meant to finish up all your homework last night, but alas, your favorite television show wound up getting your undivided attention instead.
Letting out a breathy alas in the middle of your sentence may sound dramatic or old fashioned, but it's actually an excellent way to emphasize extreme bewilderment, regret, concern, or woe. Once an expression of weariness rather than grief, alas stems from the Latin lassus, meaning “tired, weary.” Exclaimed Lord Byron, the English poet, “Alas! The love of women! It is known to be a lovely and fearful thing!”