Legend has it that the term flat-hat originated with an incident back in the days of barnstormers in which a pedestrian's hat was crushed by a low-flying airplane. According to one version of the tale, the reckless pilot was subsequently required to purchase a new hat for the hapless pedestrian. It seems unlikely that such an event actually took place, but we can well imagine how fear of having one's hat smashed flat by a passing airplane might have given rise to such a vivid verb. Flat-hat is first known to have appeared in English in 1940. Another word for flying low to the ground, the verb hedgehop, debuted at least 14 years earlier (and its related gerundhedgehopping is known to be a bit older still).