I'm a Dutchman

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.
See also: Dutchman
Farlex Dictionary of Idioms.

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.
See also: Dutchman
Farlex Partner Idioms Dictionary
See also:
  • as one does
  • as you do
  • damn well
  • count in
  • be (flat) on (one's) back
  • come in
  • come in a certain position
  • come on in
  • flat on (one's) back
  • flat on back
References in classic literature
"Not much o' fellow-creaturs, I think, Miss; all I know--my old master, as war a knowin' man, used to say, says he, 'If e'er I sow my wheat wi'out brinin', I'm a Dutchman,' says he; an' that war as much as to say as a Dutchman war a fool, or next door.
The Mill on the Floss
FURTHER to your question about the origin of the phrase 'I'm a Dutchman' in last week's Write Stuff, it originates in Anglo-Dutch enmity during the 17th century because of trade disputes and naval embargoes.
WRITE STUFF: Reader's letter
Did Ruud van Nistelrooy leave Maine Road on Saturday and say "If we just lost 3-1 to Manchester City then I'm a Dutchman"?
Sport on TV: pick of the day