The Spanish word 'graso' (meaning 'fatty') comes from the Vulgar Latin word 'grassus', meaning 'fat, thick'. 'Grassus' itself developed from two Latin sources: the Classical Latin 'crassus' (meaning 'fat, thick') and Late Latin 'grossus' (meaning 'thick'). The evolution from 'crassus/grossus' to 'grassus' to 'graso' shows a typical pattern of consonant simplification and vowel preservation common in the development from Latin to Spanish.

A common related Spanish word is 'grueso' (meaning 'thick'), which also derives from the Latin 'grossus'. This shows how the same Latin root evolved into two Spanish words with related but distinct meanings: 'graso' focusing on fattiness or greasiness, and 'grueso' emphasizing thickness or bulk.

English speakers might recognize the connection to words like 'gross' and 'crass', which both come from the same Latin roots. 'Gross' comes from Latin 'grossus' and retains meanings related to bulk or thickness (as in 'gross weight'), while also developing additional meanings. 'Crass' comes from Latin 'crassus' and has evolved to mean 'crude' or 'unrefined' in English, though it originally referred to thickness or coarseness.