Likely a modern French-style form related to Celise or Selene, giving a soft, refined sound.
Selyse is a variant spelling operating in the tradition of medieval and early modern French and English feminine names derived from the Latin Caecilia or the Greek Selene — names built on roots meaning either "blind" (in the Cecilia lineage) or "moon" (in the Selene lineage). The exact orthography Selyse has the feel of a Norman or Angevin scribal rendering, where the substitution of y for i and the final silent e reflect the spelling conventions of medieval French manuscripts. Names like Selisse, Celise, and Selyse circulated at the edges of courtly naming practice in medieval England and France, close relatives of the more common Celia, Cecily, and Céleste.
R. Martin's A Song of Ice and Fire series, where the name is borne by the rigidly devout wife of King Stannis Baratheon. Martin is known for mining medieval European naming conventions to create names that feel plausible within his quasi-medieval world without being outright historical, and Selyse fits that template exactly — it reads as a genuine name from another era without being easily traced to a single source.
The HBO adaptation Game of Thrones brought the name to wide international attention from 2012 onward. Parents drawn to Selyse today often appreciate its visual elegance — the y lends it a slightly archaic, manuscript quality — combined with its phonetic gentleness. It avoids the over-familiarity of Celia or Selene while belonging unmistakably to the same family of luminous, moon-touched feminine names. It is a name that feels both old and fresh simultaneously.