Why Is It Called Your Flies

The flies are the flap of cloth that hides the fastening method at the front of the trousers, the fastening may be buttons or a zip. The opening at the front of trousers was called a fly or more commonly flies long before the zipper existed.

Why do they call them flies?

Why is the opening on men’s trousers called a “fly”? … “Fly” also came to mean “something attached by one edge,” like a flag or banner flying from a rope or pole. With this meaning in mind, 19th-century tailors used the term “fly” for a flap of cloth attached at one end to cover an opening in a garment.

What's the meaning of your fly is open?

Apparently, when a gentleman has forgotten to zip his pants, in the US they remind him thusly. Your fly is open. Dictionary.com lists the noun fly meaning: 20. a strip of material sewn along one edge of a garment opening for concealing buttons, zippers, or other fasteners.