The word jeans is widely linked to Genoa, the Italian port city. In French, Genoa was called Gênes. A sturdy cotton cloth associated with that place seems to have been called jean or janne, and English eventually developed the plural form jeans for the trousers.
This is why two clothing words that often appear together have different histories. Jeans points to a place name, while denim comes from French de Nîmes, meaning a fabric linked to Nîmes.
- jeans: linked to Genoa, a city name
- denim: linked to Nîmes, a fabric name in French
So when we say blue jeans, we are combining a garment name with a fabric name. Not every pair of jeans must be made from denim today, but the two words became closely connected through clothing history.