Severn

From the name of the River Severn, which is of unknown meaning. The Severn is Great Britain's longest river, flowing from Wales through much of western England to the Bristol Channel. It is one of Britain’s most ancient river names, recorded as early as the 2nd century AD in the form Sabrina; its original meaning may have been "slow-moving" or "boundary".

From a medieval personal name derived from Severinus (Latin).