A short gun or firearm, with a large bore, capable of holding a number of balls, and intended to do execution without exact aim.

Smoothbore firearm with a bell mouthed muzzle and of large calibre, introduced into England in the late 16th century, probably from the Netherlands. Muzzles were either round or oval shaped and were designed to scatter the charge when fired. Not generally used for military weapons, blunderbusses were carried for protection against thieves and as such were used as late as 1840.

a short musket of wide bore with a flared muzzle

A stupid, blundering fellow.