According to the website www.weddings.co.uk the rhyme originated in Victorian times although some of customs referred in it are much older. The "something old" represents the couples friends who will hopefully remain close during the marriage. Traditionally this was old garter which given to the bride by a happily married woman in the hope that her happiness in marriage would be passed on to the new bride. "Something new" symbolises the newlyweds' happy and prosperous future. The "something borrowed" is often lent by the bride's family and is an item much valued by the family. The bride must return the item to ensure good luck. The custom of the bride wearing "something blue" originated in ancient Israel where the bride wore a blue ribbon in her hair to represent fidelity. The placing of a silver sixpence in the bride's shoe was to ensure wealth in the couples married life.
Andy