Noun
pantser (plural pantsers)
(slang, writing) An author who writes their story without a plan or an outline.
Coordinate term: plotter
I've been a "pantser" for a long time, but I'm trying to change that. Source: Internet
You all know I’m a pantser (ie, I write by the seat of my pants, with nary a clue as to where I’m headed but trusting that I’ll get there), but there’s always a part of me that wants to be organized. Source: Internet