"Tyrannosaurus" means "tyrant lizard" and "rex" is Latin for "king."
VOA: special.2009.09.01
The first meager fossils were found in Montana by dinosaur hunter Barnum Brown, who shipped them back to the American Museum of Natural History, where the new species was promptly dubbed Tyrannosaurus rex--"king of the tyrant lizards"--by museum president Henry Fairfield Osborn.