E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Im a Dutchman if I do. A strong refusal. During the rivalry between England and Holland, the word Dutch was synonymous with all that was false and hateful, and when a man said, I would rather be a Dutchman than do what you ask me, he used the strongest term of refusal that words could express.
If not, Im a Dutchman, means, I will do it or I will call myself a Dutchman.
Well, Im a Dutchman! An exclamation of strong incredulity.