caliber (n.) Look up caliber at
1560s, "degree of merit or importance," a figurative use from Middle French calibre (by mid-16c., perhaps late 15c.), often said to be ultimately from Arabic qalib "a mold for casting." Barnhart remarks that Spanish calibre, Italian calibro "appear too late to act as intermediate forms" between the Arabic word and the French. But English Words of Arabic Ancestry finds that "that idea comes with no evidence and no background historical context to support it. It is far more likely that the word was formed in French from Latin qua libra = 'what balance, what weight.'"

Meaning "inside diameter of a gun barrel" is attested from 1580s.