So, when you break down the words:
* Dinosaur: "Dino" means "terrible" or "fearsome", and "saur" means "lizard". So, dinosaur means "terrible lizard".
* Thesaurus: "The" is a combining form meaning "beast" or "animal", and "saurus" means "lizard". So, thesaurus means "beast lizard".
The word "saur" is often used in scientific names for reptiles, particularly extinct ones, to indicate their lizard-like features or ancestry.