I am surprised to see this word classed as archaic by the OED, but it’s their call.  This interjection expressing grief is related to the word “lassitude” and the obsolete “a-” interjection particle indicates admiration, surprise, or invocation.

• 14.027 Alas that he is lost!’
• 17.041 Alas!

“a, int.1.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.

“alas, int. and n.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.