The fifth pocket of your jeans, located inside the front right one (right below the waistband) is the traditional
place for storing a pocket watch—if you sport that. This pocket is more often used
for spare change, a single
key, a ticket or some other important scrap of paper.
Before blue jeans became de rigueur, gentlemen wore three piece suits. Tailors first made fob pockets on the waistcoat. It is not uncommon for waistcoats to still sport either a functional fob pocket or one sewn shut.
Despite not being a wide open space, you can stash a fair bit of crap in there—a lawyer or maybe a matrix, but not quite a poet.
About ten or more years ago I saw a television commercial wherein a young man leaves his dorm to go to a pharmacy for condoms. He does not have enough dough to get the multipack and so settles on just one. The camera shows a close-up of the young man slipping the condom into his fob pocket after which he exchanges knowing looks with the pharmacist behind the counter. In the next scene he is picking up a girl from a house which just so happens to belong to the pharmacist. The commercial ends there, leaving the viewer to guess what happens next. There is no sound save for a catchy jazz number.