I. phrasal
: in motion: as
a. : continuously active : very busy
on the fly all day long
b. : on the point of or in the course of departure
answered on the fly
c. : while still in the air : before striking the ground
caught the ball on the fly
II. phrasal
1. : in a hurry and often without preparation : hastily ; also : spontaneously
2. : simultaneously with a more central task