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.


  1. Are the terms "fly" and "flies" interchangeable? Are Americans and British generally aware of the two expressions?

  2. What is the origin of "fly" and "flies" (i.e. men's zippers)?


4 Answers 4


In Britain the term was always flies, as in your flies are undone.

The only people I have heard refer to a fly in this regard are Americans. However the two expressions can sound the same, and the difference not be apparent, since an American might say your fly's undone which sounds a bit like the British term flies.

I think the British expression goes back to the days before zips came in (1960s?) when they were always buttons - hence suggesting a plural rendering, flies being short for fly buttons.

I'm not old enough to remember anyone saying "there's a star in the east", the usual modern euphemism is to tell the unfortunate he is "flying low".

I think Americans had zips before we did, the British male for a time considering them too effeminate, before they caught on. ("Real men don't wear zips in their clothing" may have been the sentiment.)

The OED entry with examples follows. It draws no specific distinction between British and American usage, but from the examples it is fairly clear which ones are which.

  1. Something attached by the edge. Cf. flap n. 4.

    a. A strip or lap on a garment, to contain or cover the button-holes; hence something used to cover or connect (see quot. 1884). spec. (frequently in pl.) the piece of cloth that hides the fastening at the front of a pair of trousers; also, the fastening itself.

1844 Queen's Regulations & Orders Army 154 [Trousers] Open in front, with a Fly and Five Buttons.

1884 E. H. Knight Pract. Dict. Mech. IV. 351/1 Fly, the fore flap of a bootee. A strip of leather which overwraps the front vamp and receives the strings or other fastening.

1941 I. Baird He rides Sky 234 A pair of tennis shorts with zipper fly.

1942 E. Paul Narrow Street i. 6 The professor..turned toward the pissoir, unbuttoning his fly en route.

1952 ‘Vigilans’ Chamber of Horrors 27 The words button one's fly are offensive only to the prurient.

1953 M. Dickens No More Meadows i. 49 Champ, your flies are undone again. That boy! He'll be arrested yet.

1959 R. Fuller Ruined Boys ii. viii. 128 ‘Your flies are undone,’ said Matley primly.

  • Comments are not for extended discussion; this conversation has been moved to chat.
    – MetaEd
    Sep 8, 2017 at 21:11

Part 1 of your question: As an American, I can say that I have never heard the term flies in this context before, but it seems that the words are interchangeable as they refer to the same thing regardless of type of fly/flies. There are a number of slang terms for this in America such as "XYZ" (eXamine Your Zipper), "Your barn door's open," "zipper," etc. However, someone might say, "Your fly's open" which would sound similar, but is not the same, as this would translate to "Your fly is open".

In addition, neither I, nor anyone else I know, would make a distinction based on the type of mechanism used to keep your pants closed. Buttons, zipper, Velcro or whatever it is, it would still be called a fly or zipper.

Part 2 of your question: It appears the word "fly" was coined to indicate a tent flap in 1810. As typical in languages, the word was probably associated with the action of an open tent flap and that it flapped or would appear to fly, and bob's your uncle.

In 1844 a book about military clothing made the first recorded use of "fly" to indicate a "pants opening."

As to the origin of flies, it could also be referring to a tent flysheet that had "flies" to tie the flaps closed to the tent pole.

So, perhaps the origins are very similar, but not the same. One origin is referring to the action of the object and the other is referring to the object itself. Which makes perfect sense in how they are used when referring to something being "open" versus "undone".

Well, that's my 2p.

  • Welcome to ELU. This site prefers substantiated facts to opinion. You appear to have a couple of facts (about 1810 and 1844) - but without reference these are unsubstantiated. If you could provide references your answer would be much improved.
    – AndyT
    Sep 6, 2017 at 16:00
  • 1
    I would suspect, wrt. tents and "fly" flap, that the association was because such flaps are, largely, intended to keep flying insects (mostly flies) from entering the tent. Thus, the flap being undone actually allows flies in.
    – Makyen
    Sep 6, 2017 at 17:26
  • Your second paragraph gives the impression that someone might call a button or velcro closure a "zipper." Is that intentional? If so, it is far from universal. In my experience, "zipper" is used exclusively for closures with discrete, visible teeth and a sliding mechanism to engage/disengage those teeth. (i.e. Ziploc bags do not have zippers because they fail the teeth test.)
    – Adam
    Sep 6, 2017 at 17:36
  • 3
    @Adam I think the claim here (which doesn't seem outlandish) is that you might tell someone "your fly's open" or "your zipper's undone" even if they were wearing jeans with buttons instead of a zipper.
    – Casey
    Sep 6, 2017 at 18:27
  • It looks like the word comes from an archaic use of the word fly as being synonymous with 'flap,' according to the accepted answer to this question: english.stackexchange.com/questions/46669/… Sep 6, 2017 at 19:52

I can only speak to the first question: As an Australian I've only ever heard "your fly's undone" in person. I've heard the BrE "your flies are undone" a few times on TV and it sounds very strange to my ears, but I definitely understood the meaning without undue effort.

I'm generally pretty careful to ensure my fly stays closed, but on the odd occasion someone has had to inform me of my mistake, it's most commonly been "flying low" spoken quietly with a light cough.

Incidentally in Australia we would usually (in my experience) refer to zips as being "undone" or "unzipped", not "open". We don't tend to "close" a zip, we "do it up" or "zip it up" (though it's worth mentioning a zip can be in the state of "closed" in Australian English).

  • 3
    This Aussie concurs with all the points you've made. +1
    – NMI
    Sep 13, 2017 at 3:22

One British English euphemism is 'You've got egg on your chin'.

In most of England it would be 'your fly's open'. I can't quote sources for that, though I do have a lifetime of trouser inattention

  • 2
    'You've got egg on your chin' Perfectly British! Makes everyone else look the other way... :D Sep 6, 2017 at 20:09
  • I agree. And in the early days of zips (zippers to 'them') my taylor would ask me if I wanted "a zip fly" when I was being fitted for a pair of trousers. I notice that it is the description on jeans with the current absurd trend for metal buttons.
    – David
    Sep 6, 2017 at 22:10
  • 2
    I've always used it in the plural ("flies") and thought of the singular as mostly American, though sources indicate that both forms are used in the UK. (Cambridge has "fly, n. (UK also flies)"; Oxford has "British often flies".) However, I'm not sure that your personal or anecdotal experience can be extensive enough for you to be confident of which form is the more common in "most of England".
    – rjpond
    Sep 6, 2017 at 22:28

Not the answer you're looking for? Browse other questions tagged or ask your own question.