The phrase is from the Bible (Daniel 2: 31-40). King Nebuchadnezzar has a dream. The image that appears to him has a head of gold, breast and arms of silver, belly and thighs of brass, and legs of iron. The feet of this image are made of iron and clay. A stone hits the feet and the whole image breaks into pieces. The prophet Daniel's interpretation of the dream is that Nebuchadnezzar was the head of gold (a great king), but after him would come weaker kingdoms (like the image with feet of clay). These kingdoms would finally be replaced by the kingdom of God.