When Sheldon says "Ponfo Miran" in the Vulcan language in the TV show "The Big Bang Theory," he is actually saying "Go to hell" [source]. This phrase is a strong and dismissive statement, typically used to express frustration or anger towards someone. It is essential to understand that Sheldon's use of the Vulcan language on the show is intended for comedic effect and does not accurately reflect the language as depicted in the "Star Trek" series.
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