On the fly

This phrase originally meant 'on the wing' and probably derived from bird shooting, but in some Scottish and Northern England dialects, 'on the fly' means doing something with intent to deceive, and this usage dates from the early 19th century. From the mid-19th century its meaning had evolved to describing something in the air. For example, a catch taken in the air in baseball is taken 'on the fly' and this usage is first cited from 1851. 'On the fly' can also mean doing something on impulse, without thought, or in a hurry, and this usage also dates from the mid-19th century. See also Fly.