The name is originally Germanic, meaning "hook gun", and came from the hook under the muzzle. As the guns tended to be rather heavy and large-caliber, the hook made it possible to use a support of a forked stick, the forquine, to hold up the muzzle, and if the gun was used to defend a wall, the hook made it possible to hold it steady and took the recoil. The hook would be replaced by a bell at the muzzle by the end of the 15th c.. Note that the term is somewhat indistinct. We don't know whether it always referred to a matchlock gun of a somewhat small caliber with a shoulder stock.

I don't think there's a general agreement about precisely where it originated. The transition from short-barreled hand cannon to long-barreled matchlock guns coincided with the development of more uniform black powder in the 15th c.. A Hackenbüchse is mentioned for the first time in Braunschweig and Frankfurt in 1418, and they seem to have soon been commonly used in the German city-states for defense of city walls.

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Bert S Hall: Weapons and Warfare in Renaissance Europe

EDIT: Büchse is still the German word for gun, and also in Dutch. So, a donnerbuchse , or donderbus ( thunder gun) would become the English blunderbus.