Leith comes from Middle Welsh seith meaning “seven” which comes from Proto-Brythonic *seiθ (seven) via Proto-Celtic *sextam (seven) derived from PIE root word *septḿ̥ (seven). The modern Welsh word is saith though as far as I know neither has ever been used as a given name name. Seith has also been used as an anglicized form of seiðr, an Old Norse word meaning “witchcraft, sorcery” via Proto-Germanic *saidaz (magic, charm) derived from PIE root word *seyt- (magic). Seiðr was a type of sorcery practiced in Norse religion and was associated with the Norse deities Odin and Freya.
- Saith (Middle Welsh)
- Sayth (English)