Gwenllian f. Welsh gwen 'fair, white; blessed' + Welsh llian 'flaxen (cloth)'.
The name of a 12th C princess consort of Wales and the 14th C daughter of the last Prince of Wales.
Unlike many native Welsh names which succombed to the pressure of the invading Englishmen, Gwenllian remained a popular name in Wales, and even moved outside of the country and spread into England. The Welsh form Gwenllian was pronounced with three syllables, the stress on the second one. The English tendancy to stress the first syllable led English forms of the name to have just two syllables.