The Spanish word 'tesoro' (meaning 'treasure') traces back through Latin 'thesaurus' to Ancient Greek 'thesauros' (θησαυρός), which meant both 'treasure' and 'storehouse'. The Greek word itself derives from the verb 'tithemi' (τίθημι), meaning 'to put' or 'to place' - reflecting how a treasure is something that is placed or stored away for safekeeping. Over time, as the word passed from Greek through Latin into Spanish, the 'th' sound simplified to 't' and the ending was modified to the more typical Spanish '-oro' ending.

While 'tesoro' itself is a commonly used Spanish word, it doesn't have many etymologically related simpler words in Spanish. However, you might encounter it in related forms like the verb 'atesorar' (to treasure, to hoard) or the adjective 'tesorero/a' (treasurer).

The English word 'thesaurus' - a book of synonyms and related concepts - comes from the same Greek root as 'tesoro'. While today we think of a thesaurus as a collection of words, its name comes from the idea of it being a 'treasury' or 'storehouse' of words. This connection helps remember that 'tesoro' means 'treasure' - both words relate to storing valuable things, whether they're precious objects or precious words!