In relation to computer technology, "on the fly" describes activities that develop or occur dynamically rather than as the result of something that is statically predefined. For example, the content of a page that is sent to you from a Web site can be developed (and varied) "on the fly" based on dynamic factors such as the time of day, what pages the user has looked at previously, and specific user input. The Web server calls an application program to produce the "on-the-fly" page that is to be returned. There are several techniques for on-the-fly page development, including the server-side include , the use of cookie (information previously stored about you that is located in a special file on your hard disk), and Microsoft's Active Server Page .
In general usage, the expression often connotes a degree of haste and improvisation as in "I usually grab breakfast on the fly." This usage is somewhat similar to the expression "catch as catch can." The term also can simply connote being in a mobile or fluid situation. Our Webster's New World Dictionary reminds us that the term probably originated with bird hunting and shooting birds on the fly (rather than on the ground). The dictionary says simply ""in flight" and adds a colloquial meaning of "in a hurry."