Etymology
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From penta- + -phthong, by analogy with diphthong, triphthong.
pentaphthong (plural pentaphthongs)
- (phonetics, rare) A sequence of five vowels pronounced as, or almost as, a single syllable.
1866, Caleb Bates Josselyn, Elements of Pronunciation […], page 53:Fifty-six diphthongal combinations may be made from the eight vowel elements, four of which, though, we find to be necessarily triphthongs, two tetraphthongs, and one a pentaphthong.
1992 February 4, Graham Toal, “Vowel Raising Before Nasals”, in bit.listserv.words-l[1] (Usenet):Try saying the Dutch word Leeuwen some time. At least a pentaphthong...
1993 December 23, Bob Simmonds, “Seamed phthongs”, in bit.listserv.words-l[2] (Usenet):Someone once claimed that Pres. Coolidge ( a Vermonter) pronounced the vowel in "cow" as a pentaphthong.
2008 June 17, Ruud Harmsen, “British English "Joshua is"”, in sci.lang[3] (Usenet):Phonetically, [j] is meaningless, it's the same as an [i]. So what we have here is a pentaphthong: ui@iu.