The Art of Manliness uncovers everything about the humble pocket.

This is because the original pockets weren’t like the sewn-in pockets we know today, but rather separate bags detached from clothing. From the 15th until the mid-16th century, men and women carried essential items and currency in a pouch that was typically tied around the waist or hung from a belt. As thieves and “cutpurses” became more of a problem in the 17th century, people began to cut slits in their shirts, skirts, and pants, and tuck their pouches inside their clothing for safekeeping. This practice necessitated making the bags flatter and easier to reach into, so they would be more accessible and not create a significant bulge.

— A Man's Pockets
by The Art of Manliness