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on the fly
01
on the run or in a hurry
Grammatical Information
02
while moving or in the middle of doing something
Slang
What does "on the fly" mean in slang?
The structure is a prepositional phrase where "fly" implies ongoing motion. It is used to describe doing something while already in the middle of another activity, often requiring quick adjustments without stopping to prepare. For example, a programmer fixes a bug on the fly while software runs, or a driver recalculates a route after missing a turn. The term originated in 19th century baseball slang, referring to catching a ball before it hits the ground.
Examples
I did n't have time to make notes, so I just wrote them on the fly.
