thesaurus (n.)

1823, "treasury, storehouse," from Latin thesaurus "treasury, a hoard, a treasure, something laid up," figuratively "repository, collection," from Greek thēsauros "a treasure, treasury, storehouse, chest," related to tithenai "to put, to place." According to Watkins, it is from a reduplicated form of PIE root *dhe- "to set, put," but Beekes offers: "No etymology, but probably a technical loanword, without a doubt from Pre-Greek."

The meaning "encyclopedia filled with information" is from 1840, but it existed earlier as thesaurarie (1590s), used as a title by some early dictionary compilers, on the notion of thesaurus verborum "a treasury of words." The meaning "collection of words arranged according to sense" is attested from 1852 in Roget's title. Thesaurer is attested in Middle English for "treasurer" and thesaur "treasure" was in use 15c.-16c.; Elizabethan English had thesaurize "hoard as treasure."