The name is a Welsh and Cornish form of Solomon.
It is borne by two historical figures. One a saint who was a hermit in Cornwall. His feast is celebrated on June 25.
Another was a Welsh king known as Selyf Sarffgaudau, (Selyf the Battle-Serpent) (d. 616). He died at the Battle of Chester against the Northumbrian king, Aethelfrith. He was sometimes recorded as Selim.