'diarrhea' originates from Late Latin, specifically the word 'diarrhoea', which in turn comes from Greek 'diarrhoia', where 'dia-' meant 'through' and 'rhein' meant 'to flow'.

'diarrhea' changed from the Middle English word 'diarrhœa' (borrowed from Late Latin 'diarrhoea') and eventually became the modern English form 'diarrhea'.

Initially it meant 'a flowing through' (literally), but over time it came to refer specifically to the medical condition of excessive watery discharge of the bowels.