Having grown up in a very rural area, my mother used a number of interesting phrases and one of them was about "not teaching your grandmother to suck eggs". For the record, I have no idea whether any of my grandmothers actually could suck eggs, but I can assure you that this phrase meant that it never occured to me to offer them any advice in that life skill.

I thought it was a highly localized saying, but on a whim, I recently searched for the phrase and Wikipedia came to the rescue. They have a page on this exact phrase: Teaching grandmother to suck eggs. Who knew? Most of the opinions I found on the Internet believed that the phrase originated during the 1700's, with a few believing that it may have been even as far back as the 1400's. There is also a good discussion on the origin of the phrase over at the English Language section of Stack Exchange.

As for the meaning, it means to offer advice to one who knows perfectly well what they are doing and who likely knows more than you do on the matter at hand.

English is such an interesting language!