1560s, "degree of merit or importance," from M.Fr. calibre (14c.), perhaps ultimately from Arabic qalib "a mold, last," perhaps from Gk. kalopodion "a shoemaker's last," lit. "little wooden foot," from kalon "wood" + podos gen. of pous "foot" (see foot). Arabic also used the word in the sense "mold for casting bullets," which is the original literal meaning in English, though the earliest cited sense is a figurative one. Meaning "inside diameter of a gun barrel" is attested from 1580s.