We’ve discussed Arthur and we’ve discussed Lancelot: There was really little option for which name to consider next, it had to be Guinevere, the name of arguably the most important woman in Arthurian literature. The story of her betrayal of Arthur with Lancelot first occurs in Chrestien de Troyes, but her character appears in Welsh mythological cycles earlier; Geoffrey of Monmouth calls her Guanhumara; Caradoc’s Vita Gildae calls her Guennuuar; and Geraldus Cambrensis names her Wenneuereia. The initial element derives from Proto-Celtic *windo ‘white’, giving rise to modern Welsh Gwen and Gwyn, and the second element from Proto-Celtic *sēbro ‘demon, spectre’. The name is a cognate with Irish Fionnabhair (spelled Findabair earlier) and Cornish Jennifer.
The name was rare outside of literary contexts, even in Wales, until the popularization of the Arthurian stories. By the 16th C, in Wales, a wide variety of spellings can be found, such as Guinevere, Guinivere, Gwenhever, Gwenheyvar, Gwenhoivar, Gwenhover, Gwenhwyvar, Gwenover, Gwenwever, Gwynwever, and even the more unusual Gaynor (all in unprocessed data). In England, an early example of the name occurs in the matronymic byname (i.e., a descriptive byname identifying the bearer’s mother’s name), Jeneuer, which is dated to 1296 in Reaney & Wilson, s.n. Jennifer. The same source gives Gwenhevare 1431 and Jenefer 1554.
The name took on an unusual form when it moved to Italy and Iberia: In Rome it is spelled Ginevra; in Naples Genefra; in Perugia Genevria and Ginevria; in Florence Ginevera and Ginevra. In in-process data from 16th C Lisbon, we have Genebra, Jenebra, and Jenevra.