Jeans as they’re recognized today were originally work pants for American cowboys, miners, factory workers and others way back in the latter decades of the 1800s. The earliest jeans only had four pockets, but the curious fifth one is even present on the oldest pair of jeans in the Levi’s archive, dating to 1879. https://www.gearpatrol.com/watches/a710610/further-details-jeans-watch-pocket/