noun
A somewhat colloquial or everyday term for diarrhea. Often used in casual conversation.
昨日から下り腹で、何度もトイレに行っている。
I've had diarrhea since yesterday and keep going to the bathroom.
冷たいものを食べ過ぎて下り腹になった。
I ate too much cold food and got diarrhea.
下痢 is the standard medical and formal term for diarrhea, while 下り腹 is more colloquial and everyday.
Compound of 下り (くだり, 'descending') and 腹 (はら, 'belly'), literally 'descending belly', referring to loose bowels.