A heavy downpour. The origin of this expression, which has been around since at least 1700, has been lost. Among the theories are that a raging storm is analogous to fighting cats and dogs; that in North European myth cats supposedly influence weather and dogs symbolize wind; and, currently considered the most likely one, that with the primitive drainage systems in use in the seventeenth century, a heavy rainstorm would cause gutters to overflow with a torrent of debris that included garbage, sewage, and dead animals. Among the earliest appearances in print is a note by Jonathan Swift in 1738: “I know Sir John will go, though he was sure it would rain cats and dogs.”