Definition from Wiktionary, the free dictionary
- This word is incorrectly formed. For masculine nouns in the nominative case of Latin’s second declension (of which thēsaurus is one), -us and -ī are singular and plural endings, respectively; one or the other attaches to the noun’s stem (thesaur-), depending on number: the plural ending does not concatenate as thesaur- + -us + -i = thesaurusi, but rather supplants the singular ending (-us) as thesaur- + -i = thesauri — thus forming the correct Latin nominative plural, which is also valid in English. Alternatively, in English, thesaurus, can be suffixed with the the English plural suffix -es, to form another correct plural form of thesaurus, thesauruses. Thesaurusi is a rare error.