Hard boiled fiction is a tough, no nonsense world of crime, violence, and sex. Its populated with characters who talk as hard as they hit who are planning crime as often as they're fighting it.
The term hard boiled was first used in cooking recipes to describe hard boiled eggs. It eventually came to mean someone who was 'cynical, stoic, and emotionally untouchable.' They got that way through living on the city streets where the harsh reality surrounding them meant they had to harden themselves.
The idea of a tough, stoic hero has developed from cowboy fiction and mythology. In the second half of the 19th century, pulp novels were incredibly popular. They were so called because they were printed on cheap, pure wood pulp paper. The stories usually told of the West or of War, and normally featured a young man as the lead character who must cope with a strange environment. The only way to survive is to adapt quickly.
Toward the start of the 20th century, the story lines started to take on more urban settings and the fight against crime. Some based themselves on real life murders and despite only being around eighty pages long, managed to fit in an extraordinary amount of violence. The distinction between the western and crime pulps became increasingly noticeable around 1910 to 1920, with more and more crime titles available. Perhaps the most famous of these was The Black Mask which was home to writers who would eventually epitomize the genre.