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 uncle's tongue); and 2 Sat. iii. 88, “Ne sis patruus mihi”
(Don't come the uncle over me).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894