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1.
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.
de Doyman. 7 octobre 2007
 
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.
de Eptomologizzle 10 mars 2005