What is "diphthongize"

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.

vb. 1 (context transitive phonetics English) to change to a diphthong (qualifier: as by inserting or removing a vowel) 2 (context intransitive phonetics English) to become a diphthong

v. change from a simple vowel to a diphthong; "This vowel diphthongized in Germanic" [syn: diphthongise]