Fly by the seat of your pants

In modern usage means to decide a course of action as you go along, using your own initiative and perceptions rather than a pre-determined plan or theory.

On the other hand, the original meaning of the phrase comes from early aviation parlance. Aircraft initially had few navigation aids and flying was accomplished by means of the pilot's judgement.

n the one hand, we have this largely negative connotation of the phrase implied by the first definition. Most people would interpret this to be a synonym for "underprepared", "spontaneous", or "chaotic". Yet, the original meaning doesn't mean this at all. It's a very positive thing. A pilot would use his training, judgement, and skill to bring about a favorable outcome. It didn't indicate a lack of planning -- in fact, quite the opposite. A skillful pilot has to have spent a great deal of time in preparation for a mission, and yet in pre-instrument days, would also have to have an excellent sense of direction, and a knowledge of terrain and problems so thorough as to be little obstacle except in the most extreme circumstances.