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.
But in the UK a trouser zipper is also called flies,
Your flies are open
Huffington Post, edition UK, has the following title in their light-hearted article dealing with the social faux pas:
“Your Flies Are Undone!”
The article then mentions a British euphemism that was commonly heard before WWII that signalled the gentleman to fasten the buttons on his trousers.
“There’s a star in the East”
The British writer, and The Times journalist Philip Howard, seems to confirm the validity of this phrase.
Are the terms "fly" and "flies" interchangeable? Are Americans and British generally aware of the two expressions?
What is the origin of "fly" and "flies" (i.e. men's zippers)?