En l'air, au passage. Je lui jetai ma bourse, il la saisit ŕ la volée.
Doing a task or process as needed without suspending or disturbing normal operations. For example, it is often said that an HTML document can be edited on the fly because its content can be revised without the need to completely shut down or re-create the Web site on which it resides. See also HTML document, Web site.
On the run or in a hurry.