Sometimes when you are writing you want to use a word to describe something but even the deepest dive into Roget's seas leaves you with nothing. So you make one up.
Authors have done this for probably as long as there have been authors (Ookla: Me want to describe for others how you smear colors of paint on wall to represent creatures in our era but me lack pithy verb to do so. Mock: Why not say "painting?" Ookla: Brilliant! Mock: And it can describe result of action as well, being both verb and noun! Ookla: Uh-oh...)
But not every author who neologizes and portmanteaus his or her way through their text creates a phrase that not only they, but everyone else starts to use. William Shakespeare is probably one of the best known in the field, but he has many surprising companions. Paul Dickson's Authorisms goes through a few of them and presents short essays about some of the words' origins.
Where a few of our best-known words came from is rather surprising. The world "Martian," for example, is probably about 750 years old in the English language, coming to us from Geoffrey Chaucer. But it didn't start getting used as a noun, denoting an inhabitant of the Red Planet, until the late 19th century. Before that, it was mostly an adjective, describing something that might, it was thought, be found on Mars.
This is really not a mysterious process -- it happens in pop culture today, although the origination of some of the words is a lot more obscure. Although the word "selfie," for example, is almost ubiquitous today, no one really knows who first began using it to describe a picture take of oneself with the camera on one's telephone or some other digital device.
Which is probably better for that person or persons' health and well-being, anyway. Who would want that sin on their conscience?