feet of clay

When I used the idiom, "feet of clay," in class yesterday I got back looks of incomprehension.  I could easily say that the idiom indicates that a person is not as good as one might have thought, but I did not know the origin of the phrase.  Here it is: Daniel 2: 34.  The reference is to a statue made of metals, but whose feet are clay and easily broken.  So, the phrase indicates a hidden flaw that weakens the figure.