Why do they call a man's zipper a fly?

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Tags

1. #flaps
2. #called
3. #fastening
4. #originally
5. #opening
6. #blows
7. #button
8. #england
9. #flies
10. #pants
11. #refers
12. #specifically
13. #garment
14. #which
15. #material
16. #zipper
17. #refer
18. #covering