The origin of the name Lostwithiel is a subject much debated. In the 16th century it was thought that the name came from the Roman name Uzella, translated as Les Uchel in Cornish. In the 17th century popular opinion was that the name came from a translation of Lost (a tail) and Withiel (a lion), the lion in question being the lord who lived in the castle.
Current thinking is that the name comes from the Old Cornish Lost Gwydhyel meaning “tail-end of the woodland”. The view from Restormel Castle looking towards the town shows how this may have come to be.