Feet of clay is a reference to the interpretation of the dream of Nebuchadnezzar, King of Babylon by the prophet Daniel which is recounted in the Book of Daniel:[1]
Thou, O king, sawest, and behold a great image. This great image, whose brightness was excellent, stood before thee; and the form thereof was terrible.
This image's head was of fine gold, his breast and his arms of silver, his belly and his thighs of brass,
His legs of iron, his feet part of iron and part of clay. (Daniel 2:31-33)
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And whereas thou sawest the feet and toes, part of potters' clay, and part of iron, the kingdom shall be divided; but there shall be in it of the strength of the iron, forasmuch as thou sawest the iron mixed with miry clay.
And as the toes of the feet were part of iron, and part of clay, so the kingdom shall be partly strong, and partly broken.
And whereas thou sawest iron mixed with miry clay, they shall mingle themselves with the seed of men: but they shall not cleave one to another, even as iron is not mixed with clay. (Daniel 2:41-43)
The analogy is now commonly used to refer to a weakness or character flaw, especially in people of high station.[2]