The word arquebus, (sometimes spelled harquebus, harkbut or hackbut from the Dutch word haakbus, meaning "hook gun" or "hook tube," refers to an early muzzle-loaded firearm used during the 15th to 17th centuries. The Germans knew it as Hakenbüchse, or haquebute, the Italians as archibugio; which gave arquebuse (French), arcabuz (Spanish), arcabus (Portuguese) and arquebus (English).
The caliver, an improved version of the arquebus, was introduced in the early 16th century. The word is derived from the English corruption of calibre as this gun was of standard bore, increasing combat effectiveness as troops could load bullets that would fit their guns (before, they would have to modify shot to fit, force it in, or cast their own before the battle).
In the early 16th century, the term "arquebus" had a confusing variety of meanings. Some writers used it to denote any matchlock shoulder gun, referring to light versions as caliver and heavier pieces fired from a fork rest as musket. Others treated the arquebus and caliver synonymously, both referring to the lighter, forkless shoulder-fired matchlock.
As the 16th century progressed, the term arquebus came to be clearly reserved for the lighter forkless weapon. When the wheel-lock was introduced, wheel-lock shoulder arms came to be called arquebuses, while lighter, forkless matchlock and flintlock shoulder weapons continued to be called calivers.
The arquebus was used against enemies wearing steel-plate armour, standard in European combat from about 1400 until the middle of the 17th century. Plate usually stopped an arquebus ball at long range but at close range often pierced the armor of knights and other heavy cavalry. Good quality in both gun and armour was vital. The arquebus led to the development of thicker plate armour, and its final retirement.