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A vowel combination consisting of a weak vowel and a strong one.

It is more commonly used as an insult, seeing as it is a legitimately funny word.
There is a diphthong in "loud."

YOU'RE A DIPHTHONG.
by Doyman. October 07, 2007
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2
A small article of clothing worn by a homosexual man under a swimsuit or alone. Especially used at nighttime.
When I went swimming with Peter, he was wearing the sexiest diphthong.
by Eptomologizzle March 10, 2005
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