I'm a Dutchman

Used to highlight something that one finds to be extremely unbelievable or completely unacceptable. If used in the construction "or I'm a Dutchman," it indicates that the speaker thinks it is very unlikely that what they said will prove to be untrue. If this player is considered to be the best in the league, then I'm a Dutchman. She's going to win the award for Best Actress or I'm a Dutchman. They're down in this game 10–0. If they come back and tie it, I'm a Dutchman.

I'm a Dutchman

used to express your disbelief or as a way of underlining an emphatic assertion. British

1994 Ian Botham My Autobiography I read somewhere that Warne said he had been possessed by demons. Well, in that case I'm a Dutchman.