The thagomizer, or tail spikes, is an arrangement of four to ten spikes on the tails of particular dinosaurs, of which Stegosaurus stenops is the most familiar. The tail arrangement is believed to have been a defensive weapon against predators.
The term “thagomizer” was coined by Gary Larson in a 1982 Far Side comic strip, in which a group of cavemen in a faux-modern lecture hall are taught by their caveman professor that the spikes were named “after the late Thag Simmons”. The term was picked up initially by Ken Carpenter, a palaeontologist at the Denver Museum of Nature and Science, who used the term when describing a fossil at the Society of Vertebrate Paleontology Annual Meeting in 1993.
Thagomizer on Wikipedia.