The thagomizer is the arrangement of four to ten spikes on the tail of particular dinosaurs, like the famous Stegosaurus, in the clade Stegosauria. The tail is believed to be a defensive weapon against predators.
The term “thagomizer” originated as a joke from a Far Side comic strip by Gary Larson, in which a group of cavemen in a lecture hall are taught by their caveman professor that the spikes were named in honor of the late Thag Simmons (the implication being that the Thagomizer was responsible for Thag’s death, along with the typical Larson out-of-context joke that suggested cavemen might have modern surnames). ** Whatever was the original word for the stegosaurus’ spiked tail, if it indeed ever had one, “thagomizer” has, since the Far Side publication, been adopted as a genuine anatomical term, and is used by multiple palaeontological authorities, including the Smithsonian Institution.**