Of American-English origin, the term Dutch uncle denotes a person giving firm but benevolent advice.

The precise underlying notion in the choice of Dutch is unclear. The word might allude to Calvinistic sternness, or indicate a lack of genuineness as in Dutch courage. (The theory that it refers to the absence of blood relationship is not supported by any other usage of the word.)

The earliest instance of Dutch uncle that I have found is from The Boston Morning Post (Boston, Massachusetts) of Monday 26^th May 1834, which gave an account of the trial of Bill Fessenden, an escaped convict, who was arrested “while delivering an experimental lecture to an uproarious mob, upon some abstruse point of predestination and free will […]. It was clear, from his orthodox doctrine, that his early religious education had not been neglected, if his morals had.”:

As the Court was preparing to sentence him, and, having good reason to expect a scorcher, he growled out, like a wounded tiger—“If you don’t show mercy to me, how can you expect the Saviour to show you any, eh?” Finding, from a few words that fell from the Court, that his “forgive our trespasses” hint was not heeded, he resumed, in an insinuating tone—“If you’ll show me a little lenity, I’ll talk to you like a Dutch uncle, I will, upon my soul.” This original piece of wheedling was as ineffectual as his bolder brow-beating, and six months, in addition to the unexpired balance of his former term, was solemnly settled upon him.

The second-earliest instance that I have found is from the Poughkeepsie Journal (Poughkeepsie, New York) of Wednesday 20^th May 1835, in a story about two drunk young men who, one night, try to perform acrobatic feats in the street:

‘Young’uns;’ remarked a passing Charley [= night-watchman], ‘if you keep a cutting didoes, I must talk to you both like a Dutch uncle. Each of you must disperse. I can’t allow no insurrection about the premises.’

A story published in the Flag of the Union (Tuscaloosa, Alabama) on Saturday 3^rd October 1835 described a person speaking

in an oracular tone, the tone chiefly attributed to Dutch uncles.

 

cf. also:
meaning and origin of ‘Dutch auction’
origin of ‘double Dutch’ and ‘High Dutch’ (‘gibberish’)
origin of ‘Dutch treat’ and ‘to go Dutch’