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.
Compare: ON THE GO, ON THE WING 2 .
on the fly 2 adj. phr. , informal Busy; going somewhere in a hurry; going about doing things. Getting the house ready for the visitors kept Mother on the fly all day. The housekeeper of our school is always on the fly.