A thagomizer is the informal term for the cluster of four spikes on the tail of stegosaurine dinosaurs, for instance, the Stegosaurus. The name was coined in 1982 by cartoonist Gary Larson (born 1950) for his comic 'The Far Side', in which he claims the spikes were named after a fictional caveman called Thag Simmons.

The term "thagomizer" was adopted by Kenneth Carpenter (born 1949), a paleontologist at the Denver Museum of Nature and Science. It has since become an informal anatomical term for the four tail spikes.

These spikes were probably used as a defensive measure against predators. They were not, however, ever used to attack humans as Larson's comic implies. Stegosaurine dinosaurs and humans did not exist in the same era. Larson jokingly wrote, "there should be cartoon confessionals where we could go and say things like, 'Father, I have sinned – I have drawn dinosaurs and hominids together in the same cartoon.'"

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