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 8, 2007
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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 11, 2005
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An absolute dipshit who wears a thong.
See that diphthong over there! What an absolute tool!
by JoeySquie April 29, 2021
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adj. When an action or statement elicits consternation in the form of that quintessential Jewish diphthong "oy!"
"Dina's OTD anecdotes are always very diphthong-worthy. As her friends, we feel the most effective way to shore up her tenuous o-girl status is through enrollment at an accredited seminary."
by The Raging Bull April 8, 2005
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