[dif-thawng-ahyz, -gahyz, -thong-, dip-]Phonetics
verb (used with object), diph·thong·ized, diph·thong·iz·ing.
to change into or pronounce as a diphthong.
verb (used without object), diph·thong·ized, diph·thong·iz·ing.
to become a diphthong.
Also especially British, diph·thong·ise.
Origin of diphthongize
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
(often passive) to make (a simple vowel) into a diphthong
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012