With that example, when you are wearing a thong and you have to hold it (poop). Sometimes you cant hold it long enough and it comes out... it cuts through the "clay". Even if you aren't actually crapping yourself its a funny reference to use when you need to use the restroom.
Katie: Hold it, we are almost there girl.
Isabel: Im seriously cutting clay right now, HURRRRRRY!
Katie: Dont shit yourself! You'll soil your D&G!