( BrE , informal ) to tell or show sb how to do sth that they can already do well, and probably better than you can
TEACH YOUR GRANDMOTHER TO SUCK EGGS
Meaning of TEACH YOUR GRANDMOTHER TO SUCK EGGS in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005