Definition of "diphthongal"

‘This,’ the colonel said […] ‘is a lampshade. It looks like an ordinary lampshade. It was taken from the bedside lamp of Frau Koch. It’s genuine human skin.’ It sounded more gruesome spoken in his diphthongal southern manner—skee-ern.

1980, Anthony Burgess, chapter 56, in Earthly Powers, Penguin, published 1981, page 456