The English word "treasure" and the reference book called a "thesaurus" are, etymologically, the same word. Both descend from Greek thēsauros, meaning "a store laid up, a treasure, a storehouse." The difference is merely one of route: "treasure" traveled through Old French tresor and was reshaped by English, while "thesaurus" was borrowed directly from Latin in its original form. One word, two paths, two very different meanings — gold in a chest versus synonyms in a book.
The Greek thēsauros is likely derived from the root of tithenai, "to put, to place," which traces back to PIE *dheh₁- ("to set, to put"). This root is spectacularly productive: it also gave Greek thēkē ("a case, chest" — as in "bibliography" and "apothecary"), Latin facere ("to do, make"), and English "do" and "deed." The treasure, at its etymological root, is simply "a thing that has been put away" — stored for safekeeping.
In ancient Greece, thēsauroi were physical structures. At the great Panhellenic sanctuaries — Delphi, Olympia, Delos — city-states built small, temple-like buildings called thesauroi to house their offerings to the gods. The Treasury of the Athenians at Delphi, built around 490 BCE to celebrate the victory at Marathon, still stands today in reconstructed form. These buildings were simultaneously religious offerings and political advertisements, showing the wealth and piety of the city
Latin borrowed the Greek word as thesaurus, and Roman authors used it for both physical treasure and metaphorical collections. Cicero wrote of a thesaurus of arguments; the word already carried the sense of "a rich collection of valuable things" beyond mere gold. This metaphorical extension is what Peter Mark Roget seized upon in 1852 when he published his Thesaurus of English Words and Phrases — not a dictionary but a storehouse of words, organized by concept rather than alphabetically.
The Old French form tresor, which English adopted after the Norman Conquest, shows the typical French treatment of Latin: the 'th' became 't' (French having no 'th' sound), the 'au' simplified, and the unstressed vowels shifted. Middle English initially spelled it tresour or tresor before the spelling "treasure" stabilized, influenced by the French pronunciation with its palatalized 's' that became the 'zh' sound (/ʒ/) in modern English.
The word generated a rich family of derivatives. "Treasury" (the place where treasure is kept, and by extension the government department managing public funds) appeared in the 13th century. "Treasurer" (the keeper of the treasure) came at the same time — the Lord High Treasurer was once one of the most powerful officers in the English government. The verb "to treasure" (to value highly, to cherish) developed in the 14th century, adding an emotional dimension to a word originally about material wealth.
The figurative use of "treasure" — calling a person "a treasure" to mean they are invaluable — dates to at least the 15th century and shows how the word's center of gravity shifted from gold to value in the abstract. "Treasure trove" (from Anglo-French tresor trové, "found treasure") is a legal term dating to the 12th century, defining the rights to discovered hoards — a concept that still matters in English common law. The enduring human fascination with buried treasure, from pirate stories to metal detectors, keeps the word's original concrete sense alive alongside its metaphorical offspring.