'decay' originates from Old French, specifically the word 'decaïr,' where 'de-' meant 'down' and 'caïr' meant 'to fall.'

'decaïr' transformed into the Middle English word 'decayen,' and eventually became the modern English word 'decay.'

Initially, it meant 'to fall or decline,' but over time it evolved into its current meaning of 'to decompose or rot.'