What does Coin Pocket mean?

The fifth pocket, also called watch pocket. Strictly functional, it sits inside the right front pocket and justifies the term five-pocket jeans. Also known as match or watch pocket.

Heddels explains Coin Pocket

Originally intended to hold a watch, the coin pocket first appeared in 1902. Since then, it has become progressively smaller, but still functional to some.

Many denim heads use this area to achieve greater atari and fades.

Additional Resources

Coin pocket on a pair of Eternal 811: