diphthongous--prophetes.ai

diphthongous, a. rare.
  (dɪfˈθɒŋgəs)
  [f. as diphthong + -ous.]
  Of the nature of a diphthong; diphthongal.

1833 Philol. Museum II. 116 Mere modulations of the vowels, or at most different diphthongous combinations.

Oxford English Dictionary