Or I’m a Dutchman

Used as an exclamation of disbelief or to emphasise the veracity or certainty of what one has said, dates in this sense from the mid-18th century. During the Anglo-Dutch wars of the 17th century and after, the last thing a xenophobic Englishman wanted to be was a Dutchman, so he would use the expression to underpin the veracity of a statement.