Why do they call a pants zipper a fly?

Related questions

1. Why do you need a subdomain?
2. Is domain squatting illegal?
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5. What are fly pants?
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7. What is the difference between a domain and a subdomain?
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Tags

1. #front
2. #cloth
3. #protective
4. #trouser
5. #fastening
6. #trousers
7. #flies
8. #method
9. #though
10. #hides
11. #called
12. #presumed
13. #etymologically
14. #related
15. #flaps
16. #buttons
17. #other