seat-of-the-pants

"Mr Trump likes to be impulsive, he likes to sort of fly off the seat of his pants," says a former adviser to President George W Bush.

seat-of-the-pants

ADJECTIVE

informal 

• Of a person: tending to act instinctively, spontaneously, or expediently. Of an activity: done on the basis of practical experience rather than technical knowledge; informal; inexact.

Origin

1930s; earliest use found in Popular Science Monthly. From seat + of + the + pants, after by the seat of one's pants.