Rancorous Carpe, who devised an elaborate trap, baited with an assortment of weapons he believed would be irresistible to Peeves, and a vast enchanted bell jar, reinforced by various Containment Charms, which he intended to drop over the poltergeist once he was in place. Not only did Peeves break easily through the giant bell jar, showering an entire corridor with broken glass, he also escaped the trap armed with several cutlasses, crossbows, a blunderbuss and a miniature cannon.
-- Pottermore: Peeves (Pm)
Definition
An old-fashioned muzzle-loading gun.
In 1876, the caretaker of Hogwarts, Rancorous Carp, laid a trap for Peeves including several weapons he thought would lure him, including a blunderbuss. When Peeves escaped, he began to shoot at the teachers and students and had to be bargained with by the Headmistress, Eupraxia Mole, before he would stop.
Commentary
Etymology
from Dutch donderbus, from donder "thunder" + bus "gun"