Is it ensure, insure, or assure?
1823, "treasury, storehouse," from Latin thesaurus "treasury, treasure," from Greek thesauros "a treasure, treasury, storehouse, chest," from root of tithenai "to put, to place." The meaning "encyclopedia filled with information" is from 1840, but existed earlier as thesaurarie (1590s), used as a title by early dictionary compilers. Meaning "collection of words arranged according to sense" is first attested 1852 in Roget's title. Thesaur is attested in Middle English with the meaning "treasure" (15c.-16c.).