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.
something that you say when you think that something someone has said as a joke may really be true or become true 'At this rate we'll be walking all night.' 'Be careful - there's many a true word spoken in jest.'