One root of the word “fly” (meaning “to soar through the air”) also implies “flapping.” Several sites revealed that tent flaps, which cover the tent’s fasteners, much like on your 501s, lent their name to fly-front pants. The first documented use of “fly” meaning “tent flap” was in 1810.

Europeans started getting into pants, as we know them, in the 16th century. Fly-front trousers were invented around 1650, although the flap didn’t gain its flighty name till later. This center-front opening was considered a little risqué, and pants closures changed between a fly and a fall-front style for the next century or two.

In 1844, a book about military clothing made the first recorded use of “fly” to mean “pants opening.”