It goes back to a Biblical reference, book of Daniel:
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/Thou sawest till that a stone was cut out without hands, which smote the image upon his feet that were of iron and clay, and brake them to pieces.
It has been used a lot since then, for something that seems impressive but is actually weak at the base. I'm not sure when "iron and clay" got reduced to just "clay", but it was by at least the early 19th century.
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/Thou sawest till that a stone was cut out without hands, which smote the image upon his feet that were of iron and clay, and brake them to pieces.
It has been used a lot since then, for something that seems impressive but is actually weak at the base. I'm not sure when "iron and clay" got reduced to just "clay", but it was by at least the early 19th century.