Fly By the Seat of Your Pants
To fly by the seat of one’s pants means to undertake an activity with little or no planning or training in the activity.
It should be immediately apparent that this is an old aviation term. Early pilots had almost no navigational instruments and much of flying an aircraft was based on the pilot’s judgment. The term emerged in the 1930’s and was used widely to describe one pilot in particular: Douglas Corrigan.
Douglas Corrigan flew from New York to Brooklyn on July 19th, 1938. The Edwardsville Intelligencer wrote, “Douglas Corrigan was described as an aviator ‘who flies by the seat of his pants’ today by a mechanic who helped him rejuvenate 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.” (source)
The newspaper felt the need to explain the phrase to their readers which, to me at least, means two things. First, it was a new phrase at this point–not known by the general American public. And second, it may have come from the UK. The use of the word “trousers” in the quote is good evidence of that.
But why do you fly by the seat of your pants? Why don’t you fly by the sole of your shoe? Or by the hat on your head? I believe that “the seat of your pants” is a euphemism for “ass.” If the phrase “fly by the seat of your pants” originated during the 1930’s it could have been uncouth to say “ass” so people substituted the “seat of your pants.” In modern parlance we compare many things to our asses and what comes out of them–traditionally in a derogatory fashion. One such idiom, “I pulled it out of my ass,” means that you quickly and hastily made something up. This is almost identical to the meaning of “fly by the seat of your pants.” I don’t have a shred of evidence to prove this but I think it makes sense.