2. Another word for diarrhea. You get the shits when you eat something that makes your feces take on a primarily liquid form.
"I sure wish I hadn't eaten all that rat shit. I think I'll have the shits for the rest of my miserable life..."
Evolved to common usage for any item of high quality. However, in typical usage, the article "the" is required to denote superiority. Without the word "the" prefacing the word "shit," a different, usually negative, meaning applies.
His new car is the shit./His car is excellent.
This joint is shit./This is poor quality marijuana.
His new car is shit./His new car is not desirable.