doxxのようなどんな単語でも探してください。
 
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.
Doyman.によって 2007年10月07日(日)
 
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.
Eptomologizzleによって 2005年03月10日(木)