Some history

Denim has become a universal phenomenon and most people are familiar with the words jean and denim. But does anyone actually know where the two words come from?

The word denim comes from the name of a sturdy fabric called serge, originally made in Nîmes, a city in South of France. Originally this fabric was called serge de Nîmes, the name was soon shortened to denim. The city Nîmes derives its name from that of a spring, Nemausus, in a village in the Roman Empire. Which again is named after the god Nemausios from Celtic mythology.

Denim was traditionally colored blue with indigo dye to make blue "jeans", though "jean" then denoted a different, lighter cotton textile; the contemporary use of jean comes from the French word for Genoa, Italy (Gênes), where the first denim trousers were made. Genoa (Genua) was a city of the ancient Ligurians. Its name is probably Ligurian, meaning "knee", i.e. "angle", from its geographical position. Or it could derive from the Celtic root genu-, genawa, meaning "mouth", or from the Latin word of Celtic origin "ianua", meaning "door".

ref. Wikipedia