A blunderbuss (from Dutch donderbus roughly translated as "thunder pipe"), was a type of firearm that used a large-caliber barrel with a flared, trumpet-like muzzle, and were designed to be loaded with shot instead of musket balls. The blunderbuss was thus the ancestor of the shotgun.
Blunderbusses could have any type of period lockwork with known examples of matchlock, snaplock, wheellock, flintlock and even caplock weapons. While almost all blunderbusses were Muzzleloaders, there are known examples of breechloading blunderbusses, though these are very rare. They could be of either carbine or pistol length: a pistol-sized Blunderbuss was sometimes called a Dragon, and such weapons became so closely associated with cavalry that the corruption Dragoon was commonly used to refer to units of horse cavalry with infantry training.
They were popular for shipboard use where their short range was much less of a factor, and long (14-18 inch barrel) blunderbusses were issued to British mail coaches for protection from highwaymen, a forerunner to the use of true shotguns as "coach guns."
The Looney Toons depiction of using the flared muzzle to load the weapon with random junk and shrapnel is not true to history: doing so would damage the weapon. Instead, it served the twin functions of increasing spread and making it easier to pour powder and shot into the weapon.