To change a simple vowel sound into a diphthong; to cause a vowel to develop diphthongal characteristics.
From 'diphthong' + '-ize' (American verb suffix). The '-ize' suffix comes from Greek '-izein,' meaning 'to make or become,' and is the standard American English convention.
When linguists say English is 'diphthongizing' its vowels, they mean our vowels are becoming more like glides—actually, modern accents are even adding extra vowels that didn't exist a century ago!
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