http://www.rhymes.org.uk/a116a-pop-goes-the-weasel.htm - Weasel is derived from "weasel and stoat" meaning coat. It was traditional for even poor people to own a suit, which they wore as their 'Sunday Best'. When times were hard they would pawn their suit, or coat, on a Monday and claim it back before Sunday. Hence the term " Pop goes the Weasel"