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Phrase(s): Many a true word is spoken in jest. [and] There’s many a true word spoken in jest.
Prov. Just because something is said as a joke, it can still be true. • Fred: Why did you make a joke about my being stingy? Do you really think I’m cheap? Ellen: Of course not, don’t be silly. It was just a joke. Fred: But many a true word is spoken in jest.
McGrawhill's American Idioms And Phrasal Verbs