How did some crime fiction come to be described as "hard boiled"?

I've never heard the term and have no idea.  

I have to spell out eveything for you! First Devo and now this.

http://www.mysterynet.com/hardboiled/

Tough stories in pop fiction. Cynical attitude used for detectives in crime novels.

Common element, "hardboiled" - a hardboiled egg is tougher than others.