The Spanish word 'grasa' (meaning 'fat' or 'grease') comes from the Vulgar Latin word 'grassa', which was the feminine form of 'grassus' meaning 'fat' or 'thick'. The Vulgar Latin 'grassus' developed from a blend of two Classical Latin words: 'crassus' (meaning 'fat, thick') and 'grossus' (meaning 'large, strong'). Over time, the initial 'c-' in 'crassus' shifted to 'g-', influenced by 'grossus', giving us the modern Spanish form 'grasa'.

A common related Spanish word is 'grueso' (meaning 'thick' or 'fat'), which comes from the same Latin ancestor 'grossus'. While 'grasa' specifically refers to fat or grease as a substance, 'grueso' refers to thickness or fatness as a physical dimension.

The English word 'gross' is a cousin of Spanish 'grasa', as it comes from the same Latin word 'grossus'. While 'gross' has taken on various meanings in English including 'disgusting' or 'total/entire', its original meaning of 'large' or 'thick' is still preserved in phrases like 'gross weight'. Another related English word is 'crass' (meaning 'crude' or 'unrefined'), which comes from Latin 'crassus', showing how the same root evolved differently in English and Spanish.