slouch hat
A slouch hat is a wide-brimmed felt or cloth hat most commonly worn as part of a military uniform, mainly Australian, New Zealander or Rhodesian militaries. It is not always worn with a chin strap and the brim may be bent up. While it resembles the cowboy hat, usage context is for military-themed outfits and settings.
Not to be confused with the boonie hat which appears to be lighter and softer.