E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
I will talk to you like a Dutch uncle. Will reprove you smartly. Uncle is the Latin notion of patruus, an uncle, severe guardian, or stern castigator. Hence Horace, 3 Od. xii. 3, Metuentes patruæ verbera linguæ (dreading the castigations of an uncles tongue); and 2 Sat. iii. 88, Ne sis patruus mihi (Dont come the uncle over me).