Clothing Answers

Why is the opening in a pair of pants called a fly?

It is difficult to say with any precision.

Answer 1. Given that birds have to flap their wings to fly, in 'tent' terminology, there is what is called a 'fly', an extra outer layer, or a stand-alone sheet, which flaps about in the wind! In turn it seems that this word was used to describe 'fly-front pants', meaning flap-fronted pants.

There were various type of front openings of pants, most now having a vertical opening, which were originally fastened with buttons covered with a flap or fly. In due course the zip was invented, and is commonly used now as an alternative to buttons. The zip may still be covered with a fly or flap, but not necessarily.

Before the vertical opening became common, fall-fronted pants were commonly worn, where the flap opened downwards from the top.

In both cases, whether the opening was vertical or 'horizontal', the fact that there was always a flap to some extent may have contributed to the adoption of the word 'fly' (short for fly-fronted) to mean the entire front opening of a pair of pants/trousers.

Answer 2. Fly can also mean 'quick/quickly'. e.g 'He flew into a rage!' = he got angry very quickly. 'Time flies,' etc. So perhaps the fly opening is a quick opening in a pair of pants, rather than going through the rigmarole of unfastening one's belt and pulling down much of the garment in order, in the case of men, just to urinate.
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