mid-12c., tresor, from Old French tresor "treasury, hoard, treasure" (11c., Modern French trésor), from Gallo-Roman *tresaurus, from Latin thesaurus "treasury, treasure" (source also of Spanish, Italian tesoro), from Greek thēsauros "store, treasure, treasure house," related to tithenai "to put, to place," from reduplicated form of PIE root *dhe- "to set, put." In Middle English also thresur, etc.; modern spelling is from 16c. Replaced Old English goldhord, maðm. General sense of "anything valued" is recorded from c. 1200. Treasure hunt is first recorded 1913. For treasure trove, see trove.
Old English hord "a treasure, valuable stock or store, an accumulation of something for preservation or future use," hence "any mass of things preserved by being deposited together," from Proto-Germanic *huzdam (source also of Old Saxon hord "treasure, hidden or inmost place," Old Norse hodd, German Hort, Gothic huzd "treasure," literally "hidden treasure"), from PIE root *(s)keu- "to cover, conceal."
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 existed earlier as thesaurarie (1590s), used as a title by early dictionary compilers, on the notion of thesaurus verborum "a treasury of words." Meaning "collection of words arranged according to sense" is first attested 1852 in Roget's title. Thesaurer is attested in Middle English for "treasurer" and thesaur "treasure" was in use 15c.-16c.