a person who criticizes or reproves with unsparing severity and frankness.
Origin of Dutch uncle
Because Dutch is used here to describe someone manifesting the opposite of warm, affectionate, typically avuncular behavior, this term is sometimes perceived as insulting to or by the Dutch. See also Dutch.
A severely censorious person, usually a man: A ''Dutch uncle'' whose every word is reproof(1838+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with Dutch uncle
A stern, candid critic or adviser, as in When I got in trouble with the teacher again, the principal talked to me like a Dutch uncle. This expression, often put astalk to one like a Dutch uncle, presumably alludes to the sternness and sobriety attributed to the Dutch.
[ Early 1800s