thesaurus

Borrowed in the 16th century from Latin thēsaurus, itself from Ancient Greek θησαυρός (thēsaurós), meaning 'storehouse, treasure.'

The word carried its old sense of a treasury or storehouse until Peter Roget published his Thesaurus of English Words and Phrases in 1852, after which its current meaning — a book of synonyms — quickly became the standard usage.

It's a doublet of 'treasure,' meaning the two words descend from the same root but took different paths into English.