Treasure and thesaurus meet in Latin thesaurus and Greek thēsauros. The everyday treasure word passed through French before reaching English. A reference work called a thesaurus preserves a more visibly classical form of the same storehouse idea.
These are selected stages, not every spelling or intermediary. The story identifies omitted links; older forms did not all disappear at once.
Sources
- Merriam-Webster · treasure
Historical forms and the borrowing route.
- Merriam-Webster · thesaurus
The earlier part of the route.