Feet of clay

What is the meaning of the phrase "feet of clay"?

To find that somebody has feet of clay means to realize that the person is phony or at least fallible. The phrase originated with the idea of looking at a statue made of something wonderful, let's say gold, and then seeing with disappointment that the statue's feet were mere clay. I think some real statues or idols were involved; maybe somebody else can fill in the history here.