The bashi-bouzouk are irregular horsemen (that is to say, forming part of the auxiliaries enlisted for a campaign) of the army of the Ottoman Empire, with non-standardized and in practice very light armament, and a discipline weak. They participated in particular in the Siege of Vienna.
The term means "broken head" (cracked) or "bad heads".
These free corps, made up of criminals and mercenaries, are sometimes more dangerous for those they are supposed to serve than for their enemies. During the Crimean War, the sultan had sent a body of these irregulars to the generals of the allied troops:the greatest precautions had to be taken against them, and most of them died of cholera in the Dobrudja.
A good general should rather rely on them for other tasks:information, reconnaissance, pursuit, occupation of the ground, etc., or abuses commonly associated with soldiers of convenience.
Sultan Osmanly replaced these bands with even more formidable troops, arming the Kurds in European style and giving them a solid military education.
Bachi-bouzouk is one of Captain Haddock's favorite "turkhead" insults.