I can guess what this means but does anyone know?

I have always understood this to be referring to a weakness or foible that leads to someone or something's ultimate downfall, and I assumed that it was derived from the image described in Nebuchadnezzar's dream, as found in the Bible at Daniel chapter 2. That image had feet partly of clay and partly of iron, and those clay feet turned out to be its undoing, in a manner of speaking.