To have diarrhea; to suffer from loose stools or increased bowel movements (colloquial expression).
昨夜からお腹の調子が悪く、朝から腹を下している。
My stomach has been upset since last night, and I've had diarrhea since this morning.
傷んだ食べ物を食べて腹を下したので、今日は学校を休んだ。
I ate spoiled food and got diarrhea, so I stayed home from school today.
薬を飲んだら腹をくだす副作用が出た。
After taking the medicine, I experienced the side effect of diarrhea.