The most common theory states that the fifth pocket is a 'watchpocket' - designed for holding a pocket watch, the chain of which can be clipped onto a belt loop. Note Webby's first entry under fob which specifically references this idea, "a little pocket for a watch." This makes the phrase "fob pocket" which is sometimes seen a bit redundant. (For an example of that usage, one that incidentally shows some skin tight "Ladies' Breeches", see http://www.tallyho.ie/ladiesgenb.htm.)

I had previously thought it was for carrying zippo lighters, which fit in there quite nicely with most makes of jeans.

I myself keep my Canadian $1 and $2 coins in there instead, but then I have to contort myself at the checkout to squeeze the little blighters out again.

This is how it went.

A young man goes into a pharmacy and purchases three latex condoms. The proprietor gives him a dirty look as he checks out, walks outside, opens the metal condom container, checks the three condoms, and stuffs the condom container into his fifth pocket.

Later, the man hops into his car and drives over to his girlfriend's house. The door opens, and he sees her dad looking over her shoulder. A series of quick cuts between dad's face at the drugstore and dad's face at the door commences, and the ad ends.