It simply goes to show that education has gone to the dogs! The term "good egg" is much older than rhyming slang and comes from the old Latin tag "Mala gallina, malum ovum" ( A bad hen, a bad egg). Obviously it impies the opposite as well.
There was also the famous "Punch" cartoon about the Curate's egg:
https://en.wikipedia.org/wiki/Curate%27s_egg...If you look at this, do read the antithesis as well!