Teaching grandmother to suck eggs

George Cruikshank (27 September 1792 – 1 February 1878) cartoon about teaching Grandma to suck eggs

Teaching (your) grandmother to suck eggs is an English language saying that refers to a person giving advice to another person in a subject with which the other person is already familiar (and probably more so than the first person).[1]

Look up teach grandma how to suck eggs in Wiktionary, the free dictionary.
  1. ^ "The Free Dictionary". Retrieved 2009-05-07..