A fly on clothing is a covering over an opening concealing the mechanism, such as a zip, velcro or buttons, used to close the opening. The term is most frequently applied to a short opening over the groin in trousers, shorts and other garments, which makes them easier to put on or take off, allows men and boys to urinate without lowering the garment and prevents exhibitionism. The term is also used of overcoats, where a design of the same shape is used to hide a row of buttons. This style is common on a wide range of coats, from single-breasted Chesterfields to covert coats.
Trousers have varied historically in whether or not they have flies. Originally, trousers did not have flies or other openings, being pulled down for sanitary functions. The use of a codpiece, a separate covering attached to the trousers, became popular in 16th-century Europe, eventually evolving into an attached fall-front (or broad fall). The fly-front (split fall) emerged later.1 The panelled front returned as a sporting option, such as in riding breeches, but is now hardly used, flies being by far the most common fastening. Most flies now use a zip, though button flies continue in use.
^Croonborg, Frederick: The Blue Book of Men's Tailoring. Croonborg Sartorial Co. New York and Chicago, 1907. p. 123