The phrase "fly by the seat of your pants" means to do something based on instinct and improvisation rather than following a set plan or relying on technical knowledge. The origin of this phrase is rooted in the early days of aviation. 

The phrase comes from the early 20th century when aviation was still in its infancy. Pilots often had to rely on their intuition and immediate reactions to fly, especially in situations where instrumentation and reliable navigation aids were limited or unavailable. "Flying by the seat of your pants" referred to the pilot's ability to feel the aircraft's movements through the seat, making adjustments based on these sensations. The "seat" in the phrase refers to the pilot's seat in the cockpit. 

Early pilots had to rely heavily on their sense of touch and the physical feedback they received from the aircraft, particularly from the vibrations and movements felt through the seat, to gauge how the plane was performing. The phrase likely gained popularity during the 1930s and 1940s as aviation technology was rapidly developing, but many pilots still had to rely on their skills and instincts. 

The first recorded use of the phrase is attributed to Douglas Corrigan, an American aviator who became famous for his unauthorized transatlantic flight in 1938. When asked how he managed the flight, he reportedly replied that he "flew by the seat of his pants."