There's a fly in my pants!

Why is the zipper on pants called a fly?

I searched etymonline for "fly" and found nothing related to pants.

Is this a particularly American usage, or more global?

  • 2
    Can't help you with the etymology but I can say that's what they are called on the right-hand side of the pond, giving rise to expressions such as "flying low" and "flying without a licence". Oct 29, 2011 at 17:00
  • "There's a fly in my pants?"!? "Waiter, what this fly doing in my soup!?" "Hmm...The backstroke, I believe, sir." Oct 29, 2011 at 18:11
  • 1
    @mickeyf ::sotto voice:: Please, sir! Keep your voice down or everyone will want one. Nov 1, 2011 at 0:37

2 Answers 2


From etymonline:

fly (v.1)
Notion of "flapping as a wing does" led to noun sense of "tent flap" (1810), which yielded (1844) "covering for buttons that close up a garment."

Wikipedia says:

In the 1930s, a sales campaign began for children's clothing featuring zippers. The campaign praised zippers for promoting self-reliance in young children by making it possible for them to dress in self-help clothing. The zipper beat the button in 1937 in the "Battle of the Fly", after French fashion designers raved over zippers in men's trousers. Esquire declared the zipper the "Newest Tailoring Idea for Men" and among the zippered fly's many virtues was that it would exclude "The Possibility of Unintentional and Embarrassing Disarray."[citation needed]

The word fly is commonly used in the UK as well.

More here.

  • 2
    +1 It’s worth noting that the first fly-front trousers appeared in 1650 or so, after leggings and codpieces went out of fashion toward the end of the 1500s. It seems that the word “fly” didn’t emerge until the early- to mid-1800s (in 1810 for a tent flap, then later by analogy for a trouser flap). It seems to be a bit of a mystery what they called non–fall-front trousers before then.
    – Jon Purdy
    Oct 29, 2011 at 17:02

One of the meanings given in the OED:

A flap is:

Something attached by the edge. Cf. flap

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. (freq. pl.) the piece of cloth that hides the fastening at the front of a pair of trousers; also, the fastening itself.

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