Only since the 1950s has the word jeans referred to a specific item of clothing: the jeans trouser with familiar features such as its stitched threads, rivets, five pockets and stitched back pockets.
From the mid 20th century, younger generations started to wear the blue pants to visibly rebel against prevailing norms and values. Jeans were therefore deemed unsuitable. This changed in the 1970s when the garment became commonly accepted.
Jeans were originally made from twill weaved cotton, which differed from denim in that both warp and weft threads have the same colour. In the mid 18th century, the current spelling of jeans started to appear in England.
The use of jeans (in plural) does not necessarily the plural form of jean, but a derived form from the French spelling Jannes or Gênes. Both uses occurred at random. First referring to a type of fabric, jeans is now associated with an item of clothing.
Source: Nouvelle de Nîmes Nº 5: The Denim Dictionary