The word caliber has two possible origins. It was definitely borrowed in the late sixteenth century from French calibre, which described the measurement of internal diameter of a gun's barrel. This was primarily used in English to figuratively describe "measurements" of personal quality or ability, hence the modern definition. Calibre is from a Spanish word spelled the same way, which is also the source of the English noun calliper (a tool used for measuring internal or external dimensions). Here's the split: some linguists say the origin of that is Arabic qalib, meaning "casting mold" (that would trace to Ancient Greek kalapous, which had to do with shoemaking), but others say there's poor evidence for that and it's actually from Latin qua libra, meaning "of what weight". Either way, a very interesting story!
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.