'fatness' originates from Old English, specifically the word 'fættnes,' where 'fætt' meant 'fat' and '-nes' was a suffix indicating a state or condition.

'fættnes' transformed into the Middle English word 'fatnesse,' and eventually became the modern English word 'fatness'.

Initially, it meant 'the state of being fat,' and this meaning has largely remained the same in modern usage.