I'm a Dutchman en · X
Etymology
Likely dates to the 17th century Anglo-Dutch wars, along with other uncomplimentary terms for the Dutch.
Meanings
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(UK, humorous) Statement said after describing or hearing something obviously false, or as an impossible alternative to something that is sure to happen.
This is going to mean a gold watch and a week's outing at Southend with full pay for both of us or I'm a Dutchman.
1934, Ernest Bramah, The Bravo of London: