on the fly
on the fly (1) {adv. phr.}
1. While in the air; in flight.
The bird caught a bug on the fly.
Joe was called out because the catcher caught the ball on the fly.
2. {informal}
Between other activities; while busy with many things.
The president was so busy that he had to dictate letters on the fly.
John is very busy, and people who want to talk to him have to catch him on the fly.
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Source: A Dictionary of American Idioms
