What is the X in X-mas?
1650s, from Dutch donderbus, from donder "thunder" (Middle Dutch doner, donder, from Proto-Germanic *thunaraz; see thunder (n.)) + bus "gun" (originally "box, tube"); altered by resemblance to blunder.
short, muzzle-loading shoulder weapon, usually a flintlock, with a wide smooth bore flared at the muzzle to a maximum width of about 4 inches (10 centimetres). The flaring was intended to scatter the shot at very close range, an effect that later scientific experiments showed did not occur. The blunderbuss, forerunner of the shotgun, was common in the 18th century, though in use somewhat earlier and later.