'Diphthongize' definitions:

Definition of 'diphthongize'

From: WordNet
verb
Change from a simple vowel to a diphthong; "This vowel diphthongized in Germanic" [syn: diphthongize, diphthongise]

Definition of 'Diphthongize'

From: GCIDE
  • Diphthongize \Diph"thong*ize\, v. t. & i. To change into a diphthong, as by affixing another vowel to a simple vowel. "The diphthongized long vowels." --H. Sweet. [1913 Webster]