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Dutch uncle
Dutch uncle is a term for a person who sternly lectures, comments or criticizes with unsparing severity and blunt frankness, to educate, encourage or admonish someone, often with benevolent intent, as though an elder relative or "uncle."

One thought is that the term originated in the early 1800s, this expression, often put as 'talk to one like a Dutch uncle', presumably alludes to the sternness and sobriety attributed to the Dutch. Some, though, have speculated that it may allude to qualities attributed to the Deutsch (Germans).

Example: "When I was late for class, the teacher talked to me like a Dutch uncle."

Another thought is that the term originated in when William of Orange married Mary II and later became the head of the British throne. When the English and Dutch navy were combined during this period the English sailors viewed the Dutch with some resentment and unwanted advice or orders from Dutch sailors was referred to as being from "my Dutch uncle".