Dutch uncle is an informal term for a person who issues frank, harsh or severe comments and criticism to educate, encourage or admonish someone. Thus, a "Dutch uncle" is the reverse of what is normally thought of as avuncular or uncle-like (indulgent and permissive).
Origins
During the Anglo-Dutch Wars between England and the Netherlands in the 17th century, the English language gained an array of insults (including "Dutch uncle"), such as:
These terms also gained prominence in 17th-century New England during its rivalry with New Netherland, which the Dutch captured (and later recaptured) during the Second Anglo-Dutch War.
These colorful (though now incongruous) phrases became part of English usage worldwide, and some are still in use today.
Alternative explanations
Another proposed explanation is that the term, often expressed as "talk to one like a Dutch uncle," originated in the early 19th century as an allusion to the sternness and sobriety attributed to the Dutch people. Dutch behaviour is defined in the book Culture Shock! Netherlands: A Survival...