When you have a very sudden and extreme need to use a toilet.
Derived from "turtle head poking out".
Denoting an unexpected, well-armed and violent turtle, a pissed-off Raphael in your stomach if you will.
A: Wow honey, that was some delicious curry and black coffee. Those green peppers sure were hot!
B: Yeah... *clutching stomach*
A: You look green. Are you okay?
B: No, I've got a ninja turtle in here. *pointing to stomach* Be right back.