Fly by the seat of one's pants

Meaning

Decide a course of action as you go along, using your own initiative and perceptions rather than a pre-determined plan or mechanical aids.

Origin

This is early aviation parlance. Aircraft initially had few navigation aids and flying was accomplished by means of the pilot's judgment. The term emerged in the 1930s and was first widely used in reports of Douglas Corrigan's flight from the USA to Ireland in 1938.

That flight was reported in many US newspapers of the day, including this piece, entitled 'Corrigan Flies By The Seat Of His Pants', in The Edwardsville Intelligencer, 19th July 1938:

"Douglas Corrigan was described as an aviator 'who flies by the seat of his pants' today by a mechanic who helped him rejuvinate the plane which airport men have now nicknamed the 'Spirit of $69.90'. The old flying expression of 'flies by the seat of his trousers' was explained by Larry Conner, means going aloft without instruments, radio or other such luxuries."

Two days before this report Corrigan had submitted a flight plan to fly from Brooklyn to California. He had previously had a plan for a trans-Atlantic flight rejected (presumably on the grounds that the 'Spirit of $69.60 wasn't considered up to the job). His subsequent 29 hour flight ended in Dublin, Ireland. He claimed that his compasses had failed. He didn't openly admit it but it was widely assumed that he had ignored the rejection of his flight plan and deliberately flown east rather than west. He was thereafter known as 'Wrong Way Corrigan' and starred as himself in the 1938 movie The Flying Irishman.

The 'old flying expression' quoted above (although it can't have been very old in 1938) that refers to trousers rather than pants does suggest that the phrase was originally British and crossed the Atlantic (the right way) prior to becoming 'flies by the seat of one's pants'.