How to say "diarrhea" in Chinese. It may be gross, but if you need to say it, there's no substitute for knowing how to say it.
(Note: I went with a stylized image to maintain some small 
modecum of decorum in this article.)
First, take the word 拉, pronounced la1, which means pull, draw, drag, tug, transport by vehicle, haul, move (as in troops to a place), drag out, draw out, space out, give or lend a hand, help, drag in, implicate, involve, draw or drag in, win over, organize, set up, put together, canvasse, solicit, chat, engage in chitchat, and finaly... to poop.
Next, you need the word for stomach, which is 肚子 (du4 zi0).
Put these together, and you get the Chinese word for "to have diarrhea", which is 拉肚子 = la1 du4 zi0.
Q.E.D.