Humans and stegosaurus missed each other by more than 150 million years, but people have always wondered how difficult or terrifying life would have been if dinosaurs and humans co-existed. This premise is often explored humorously in cartoons and in movies. Cavemen and dinosaurs frequently featured in cartoonist Gary Larson’s The Far Side— a single-panel comic that ran for fifteen years during the 1980s and 90s. The Far Side was known for its surrealistic and dark humor based on uncomfortable social situations and improbable events, including aliens, talking cows, as well as human-dinosaur interaction.
In 1982, Larson drew a comic depicting a prehistoric classroom. A caveman is giving a lecture to an audience of other cavemen. Before them is a large image of a stegosaurs’ tail. The professor points towards the spikes at the end of the tail and explains that they are called the “thagomizer”, after the late Thag Simmons.
It turns out that this arrangement of spikes at the tip of a stegosaurus’s tail had no formal name. A 2006 article in the New Scientist explains why that happened: Paleontologists don't get many chances to name new bones. Evolution uses the same bones over and over again, altering their shape and purpose but preserving their basic nature, so anatomists simply use the same old terms to describe them. A humerus is a humerus, whether it's in a chicken wing, a walrus flipper, the massive front leg of a brachiosaurus or our own upper arm.
Shortly after, the Dinosaur National Monument in Utah got the thagomizer label on its spikes, and James Orville Farlow of Indiana University included the term in his book The Complete Dinosaur. Smithsonian Institution's stegosaur fossil display also has the label thagomizer.
Since then, the word has become a semi-formal term for that part of the stegosaur’s anatomy.