1560s, "degree of merit or importance," a figurative use from Middle French calibre (late 15c.), apparently ultimately from Arabic qalib "a mold for casting." Arabic also used the word in the sense "mold for casting bullets," which is the oldest literal meaning in English. Meaning "inside diameter of a gun barrel" is attested from 1580s. Barnhart remarks that Spanish calibre, Italian calibro "appear too late to act as intermediate forms" between the Arabic word and the French.
caliber cal·i·ber (kāl'ə-bər)
The diameter of the inside of a round cylinder, such as a tube.