'antidiarrhetic' originates from Modern English formation combining the prefix 'anti-' (meaning 'against') with 'diarrhetic', where 'anti-' meant 'against' and 'diarrhetic' is ultimately from Greek 'diarrhoia' (a flowing through).

'antidiarrhetic' was formed in English by adding the prefix 'anti-' to the adjective 'diarrhetic' (itself derived via Latin/French from Greek 'diarrhoia'); Greek 'diarrhoia' passed into Latin and then into Middle English as 'diarrhoea', from which medical adjectives like 'diarrhetic' and later 'antidiarrhetic' developed.

Originally the Greek root 'diarrhoia' meant 'a flowing through'; over time it came to denote the medical condition of frequent loose bowel movements ('diarrhoea'), and 'antidiarrhetic' evolved to mean 'a drug or agent that stops or prevents diarrhea'.