Wikianswers explains that coins were made during the Iron age with a peg that had a stencil of the coin on one end. The peg was gripped in one fist and it was struck by a mallet in the opposite hand to form the face of the coin, thus leading to the expression “hand over fist.” There is an opposing theory that the expression refers to seamen pulling rope hand-over-fist. However, I’m going with the first explanation, because the expression is often expanded to “making money hand over fist.”