The current meaning of thesaurus was first attested in 1852 in Peter Roget's Thesaurus. He borrowed an existing word meaning something more like "encyclopedia" for that book title, which was borrowed in the 1500s from Latin thesaurus, which actually had a definition more like "treasure" but had a figurative meaning of "repository" through a shared meaning of "treasury". This comes from Ancient Greek thesauros, which could have been defined as "vault", "chest", "treasure", or anything along those lines. Prior to that, we can trace the word as from Ancient Greek tithemai, meaning "to place", from the Proto-Indo-European dhe, "to set". Right around the '60s, thesaurus rapidly began to increase in usage, which is correlated with increased purchases of thesauruses (a plural which is more correct than thesauri, but both variations are allowed). Well, that's that. Treasure your words.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.