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Before zips were invented trousers had buttons. Buttons resembled flies. With undone buttons your 'flies were down' and your dick likely to drop out. When zips took the place of buttons the phrase remained.
i think its to do with your zip being called your fly trap
I have never heard that expression. I have heard 'flies are undone'. That is because that part of trousers is called the flies, as in fly sheet, a canvas covering. That has been called flies since before zips were invented.
The "fly" is not the zipper. It is that little metal (or plastic) piece that is attached to the zipper to close and unclose the zipper. Therefore, if your fly is down, you zipper is open ("undone"). Simple as that.
I, too, have only heard "flies (or fly) are undone" rather than down. However, the fly, or more commonly in Britain flies, refers to the opening in the crotch of a pair of trousers, closed by buttons or a zip, and typically covered with a flap of material. A fly, in this sense, can also mean a "flap of material covering the opening of a garment or of a tent". (O.E.D.)
As Gail T says, it's 'Your flies are undone' and it has nothing to do with zips. The 'fly' was originally the piece of material covering the opening of the trousers, but came to mean the opening itself, whether button, zip or velcro driven.
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