This phrase comes from the Old Testament (Dan 2:31-32). There the Hebrew captain Daniel, interprets a dream for Nebuchadnazzr, founder of the new Babylonian Empire. Nebuchadnezzar had dreamed of a giant idol with a golden head, silver arms and chest, brass thighs and body, and iron legs. Only the feet of this image, compounded of iron and potter's clay, weren't made wholly of metal. Daniel told Nebuchadnezzar that the clay feet of the figure made it vulnerable, that it prophesied the breaking apart of his Empire. Over the years readers of the Bible were struck with the phrase "feet of clay" in the story and it was used centuries ago to describe an unexpected flow or vulnerable point in the character of a hero or any admired person.
fifth wheel:
Looking at the fifth wheel of wagons and carriages many people thought it had no function, but this wehhl or circular plate, which was attached to the upper side of the front axle and never touched the ground, supported the vehicle's body when it made a turn. Ignorance prevailed, however, and the expression of fifth wheel came to mean "a useless or needless person or thing in any enterprise".
forbidden fruit:
An ancient phrase that has it's origins in Gen. 3:3,although the exact words are not found there: "But of the fruit of the tree which is in the midst of the garden, God hath said, Ye shall not eat of it, neither shall ye touch it , lest ye die". Figuratively, forbidden fruit, is usually used to describe a tempting but forbidden person or thing.
forgotten man:
Franklin Roosevelt popularized this phrase when running for president in 1932, applying it to all those suffering during the Great Depression whom the government had done little to help. The term was invented, however, by Yale professor William G. Sumner, who in 1883 applied it to the American workingman, the decent, average American citizen.