The definition of seat-of-the-pants is slang to describe something improvised, or done based on prior experience or by feel.
If you don't prepare for a speech but you just get up improvise it, this is an example of when you are doing it by the seat-of-your-pants.
Based on or using intuition and experience rather than a plan or method.
Performed without using instruments.
A seat-of-the-pants landing of the aircraft.
(slang) Using or relying on intuition and experience rather than instruments, technology, or a predetermined plan.