“A good egg” is an old-fashioned term for a good guy or a kind person. The expression originally came from its opposite, “bad egg,” British public school slang from the 1800s for someone who was not nice. Fifty years later, “good egg” began to be used for a good person.
Literally, of course, a good egg is one that is good to eat, rather than one which has gone rotten. And it is with chicken eggs that the phrase originated. Eggshells are just the surface of the egg, the same as first impressions of people are largely based upon surface details.
These days, because of regulations, it is quite rare to crack an egg open and find that it’s rotten, but this used to be a common experience. So, when a person who seemed at first sight to be a good person but turned out to be nasty, it was natural enough to call that person a “bad egg,” while someone who proved to be honest and helpful was a “good egg.”
PG Wodehouse, who attended Dulwich in the last days of the 1800s, did much to popularize the phrase. He used it first in Something Fresh in 1915. But he wasn’t the only writer who liked “good egg.” Rudyard Kipling used it in his book Traffics and Discoveries in 1904.
Then there’s the phrase “curate’s egg.” The term refers to something that is obviously bad, but is euphemistically described as nonetheless having good features.
The phrase derives from a cartoon published in the humorous British magazine Punch on November 9, 1895. Drawn by George du Maurier and titled True Humility, it pictures a timid-looking curate eating breakfast in his bishop’s house. The bishop says: “I’m afraid you’ve got a bad egg, Mr Jones.” The curate, desperate not to offend his eminent host and ultimate employer, replies: “Oh no, my Lord, I assure you that parts of it are excellent!” (This clearly cannot be true of a bad egg.)
I’ll end with a quote from Bernard Meltzer: “A true friend is someone who thinks that you are a good egg even though he knows that you are slightly cracked.”