A bashi-bazouk (in Turkish başıbozuk, meaning one with an unsound mind) was an irregular, mounted, unpaid mercenary soldier of the Ottoman army. They were recruited from homeless, vagrants, criminals, slaves and prisoners of war. Foreign mercenaries and volunteers could also be hired to the corps. These groups, however, were negligible in comparison to the largest portion of the bashi-bazouk soldiers, who were simply raving lunatics gathered from insane asylums from all around the Empire, hence the name "bashi-bazouk".

The bashi-bazouks were a rather interesting part of the Ottoman military forces. In an offensive attack, these men would be sent out first by the Ottoman generals. Although they would usually be provided with minimal armor, they would always be fitted with an tight, iron helmet that fit their heads so snugly that it would create a most unpleasant sensation on their skulls. This was done to annoy and further aggravate them, so that they would be provided with a "fuel source" of strength and anger when fighting on the battlefield. In the event of an initial Ottoman attack, therefore, what the enemy troops would see, would be a roaring and shrieking avalanche of raving madmen on horses that attacked whatever and whoever they saw at first sight with no mercy or compassion whatsoever. These barbaric lunatics, therefore, would cause much damage, havoc and fear among the enemy army. At this point, a second attack of elite, professional troops would be sent out by the Ottoman generals, which would then easily defeat the already weakened and frightened enemy troops with minimal loss and damage.

The bashi-bazouk were notorious for being brutal, random and undisciplined, thus giving the term its second, colloquial meaning of "undisciplined bandit". Hergé's Captain Haddock from The Adventures of Tintin was especially fond of this word, commonly using it as an expletive.