seat-of-the-pants
/ˌsiːtə(v)ðəˈpants/ /ˈˌsidə(v)ðəˈpaen(t)s/informal
adjective
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.