on the fly

Concise Oxford English Dictionary © 2008 Oxford University Press:
fly1

▶verb (flies, flying; past flew; past part. flown)

• 1 (of a winged creature or aircraft) move through the air under control.

  ■ control the flight of (an aircraft).

• 2 move or be hurled quickly through the air.

  ■ Baseball hit a ball high into the air.

• 3 go or move quickly.

  ■ informal depart hastily.

  ■ (of time) pass swiftly.

  ■ (as adj. flying) hasty; brief: a flying visit.

  ■ (fly into) suddenly go into (a rage or other strong emotion).

  ■ (fly at) attack verbally or physically.

• 4 wave or flutter in the wind.

  ■ (of a flag) be displayed on a flagpole.

• 5 N. Amer. informal be successful.

• 6 archaic flee.

▶noun (pl. flies)

• 1 (Brit. also flies) an opening at the crotch of a pair of trousers, closed with a zip or buttons.

• 2 a flap of material covering the opening of a tent.

• 3 (the flies) the space over the stage in a theatre.

• 4 (pl. usu. flys) Brit. historical a one-horse hackney carriage.

– phrases
fly in the face of be openly at variance with (what is usual or expected).
fly a kite informal try something out to test public opinion.
fly off the handle informal lose one's temper suddenly.
on the fly while in motion or progress.
with flying colours with distinction.

– derivatives
flyable adjective.

– origin OE flēogan, of Gmc origin; rel. to fly2.

Concise Oxford English Dictionary © 2008 Oxford University Press:
fly2

▶noun (pl. flies) a flying insect of a large order characterized by a single pair of transparent wings and sucking or piercing mouthparts. [Order Diptera.]

■ used in names of other flying insects, e.g. dragonfly, firefly.

■ a fishing bait consisting of a mayfly or other natural or artificial flying insect.

– phrases
drop like flies die or collapse in large numbers.
a fly in the ointment a minor irritation that spoils the enjoyment of something.
fly on the wall an unnoticed observer.
there are no flies on —— the person specified is quick and astute.

– origin OE flȳge, flēoge, of W. Gmc origin; rel. to fly1.

Concise Oxford English Dictionary © 2008 Oxford University Press:
fly3

▶adjective (flyer, flyest) informal

• 1 Brit. knowing and clever; worldly-wise.

• 2 N. Amer. stylish and fashionable.

– derivatives
flyness noun.

– origin C19: of unknown origin.