Pre-Norman
Although there have been settlements in the area since before Roman times, mostly being offshoots of Portchester, Portsmouth is commonly regarded as having been founded in 1180 by John of Gisors (Jean de Gisors). Most early records of Portsmouth are thought to have been destroyed by Norman invaders following the Norman Conquest. The earliest detailed references to Portsmouth can be found in the Southwick Cartularies.
However, the Oxford Dictionary of British Place Names gives the Anglo-Saxon name "Portesmūða" as late 9th century, meaning "mouth Portus" (from Latin). In Anglo-Saxon times a folk etymology " mouth belonging to a man called Port" arose, which caused a statement in the Anglo-Saxon Chronicle that in 501 AD "Port and his 2 sons, Bieda and Mægla, came with 2 ships to Britain at the place which is called Portsmouth". It has been suggested that this is more likely to refer to the area around Portchester.
Read more about this topic: History Of Portsmouth