1
general::
Phrase(s): on the fly
[done] while something or someone is operating or moving. • I’ll try to capture the data on the fly. • Please try to buy some aspirin somewhere on the fly today.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
In a hurry, on the run, as in I picked up some groceries on the fly The transfer of this expression, which literally means "in midair or in flight," dates from the mid-1800s.
American Heritage Idioms