the word 'thesaurus' entered english in the late 16th century, directly from latin, where it meant 'treasury' or 'storehouse'. its greek ancestor, 'thēsauros', carried the same literal meaning of a place where valuables like gold or precious items were kept. the semantic shift from a storehouse of objects to a storehouse of words occurred in the 19th century, most famously with peter mark roget's publication of his "thesaurus of english words and phrases" in 1851. thus, the modern meaning logically extends the ancient concept of a curated collection, applying it to language rather than material wealth.

Structure: thesaur (treasure) + us (noun suffix) thesaur is from latin 'thesaurus', meaning 'treasure, treasury', which is from greek 'thēsauros'. the suffix -us is a latin masculine noun ending.