E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Dont come the uncle over me. In Latin, Ne sis patruus mihi (IIrace: 2 Sat., iii. 88)i.e. do not overdo your privilege of reproving or castigating me. The Latin notion of a patruus or uncle left guardian was that of a severe castigator and reprover. Similarly, their idea of a step-mother was a woman of stern, unsympathetic nature, who was unjust to her step-children, and was generally disliked.
Metuentes patruæ verbera linguæ.Horace: 3 Odes, xii. 3.