The simile is meant to be absurd. The origins of the phrase are not clear. It is quite an old phrase and is included in John Stevens' translation of Quevedo's Comical Works And was there really such a demand for empty eggshells? A distaff is just a stick for holding your wool or flax. Thank you for your interest in this question. Hackworth 1, 8
I've been sucking eggs for sixty years, think I don't know you need two holes?
Teaching grandmother to suck eggs
The spinning version makes perfect sense: Another early example, whimsically inverted, is in Tom Jones by Henry Fielding, published in In fact "grandma" is more prevalent on Google. We say likewise, teach your Granny to grope her Goose. This one is well documented.