Entry 1 sense Webster, 1913 Diphthongize · Diph·thong·ize · IPA /ˈdɪfθɑŋaɪz/ 01 v. t. & i. To change into a diphthong, as by affixing another vowel to a simple vowel. 1. To change into a diphthong, as by affixing another vowel to a simple vowel.