a Dutch uncle
One who addresses someone severely or critically. Fred is always lecturing me like a Dutch uncle, forgetting the fact that I'm 40 years old!
a Dutch unclea kindly but authoritative figure.
Dutch here probably means no more than that the person described is not a genuine blood relation. In the mid 19th century I will talk to him like a Dutch uncle (meaning ‘I will give him a lecture’) was noted as being an American expression.
1999 Daily Telegraph She was the kindest of Dutch uncles, always prepared to listen to one's troubles.