The idea is all about trying to teach someone how to do something they are an expert in. (Like a newbie telling a guru how to program.)

I'm not sure, but I think the phrase comes from Easter Eggs - the women would make a small hole in the shell, and would suck out the insides of the eggs (with a straw, you moron), replacing it with chocolate, and later decorating the egg.

Both "don't teach your grandmother to suck eggs" and "teach your grandmother to suck eggs" are in common use, with the longer form meant to dissuade the listener from trying to instruct the speaker on some activity in which the speaker is more accomplished. The shorter form is meant to advise the listener that his intended instruction to a third party is, likewise, unnecessary. Both forms originate in a time when false teeth were less common than they are now, and raw or soft-boiled eggs were a preferred form of nourishment for many older people.

A little something extra:

Teach your grandmother to suck eggs. Attempting to teach your elders and superiors. The French say, “The goslings want to drive the geese to pasture” (Les oisons veulent mener les ois paitre).

from Dictionary of Phrase and Fable; 1898; E. Cobham Brewer (1810–1897)

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