A thagomizer is the cluster of large, bony spikes at the end of a stegosaurus's tail. These pointed spikes, usually four of them, could measure up to three feet long and would have been a formidable weapon against predators like Allosaurus. When a stegosaurus swung its thick, muscular tail, those spikes could deliver a devastating blow.

The word comes from a surprisingly funny source: cartoonist Gary Larson used it in 1982 in his comic strip The Far Side. In the cartoon, a caveman lecturer points to a stegosaurus tail and explains that the spikes are “named after the late Thag Simmons.” The joke, of course, is that humans and dinosaurs never actually lived at the same time. But paleontologists liked the term so much that they started using it in scientific papers and museums.