A good egg might be something you order for breakfast, but it could also be the person who’s sitting across the table. Such a person is one heck of a decent human being. There are lots of people running around the globe who are reasonably good folks, but that’s not enough to be called a good egg.
This phrase refers, first and foremost, to someone who is utterly reliable and trustworthy. Good eggs don’t tell secrets, especially embarrassing ones. They show up when they say they will, especially when the thing they’re showing up for is more about providing support to someone else than having fun themselves. Good eggs probably won’t ask to borrow money in the first place, but if they do, the lender can be certain of getting repaid on time and will no doubt also be handed a gift certificate to a really nice coffee shop.
Everybody likes a good egg, not necessarily because of a scintillating personality but because he or she has always got a buddy's back. Good eggs don’t have big egos. He or she is likely to be the quiet worker in the office who turns in work early in order to help someone who is struggling and doesn't mind when the boss praises that worker’s efforts and not his or her own. This isn’t to say that good eggs are pushovers, however. They are emotionally sturdy, which means their good-hearted actions aren’t attempts to forge obligation.
For every good egg, there’s a bad one. Bad eggs are always out for themselves. Everything they do, even if on the surface it appears to be for the benefit of others, comes right back to their own bottom lines. Lend a bad egg money, and it will be repaid in emotional manipulation designed to make the lender feel badly for expecting to see the money again. Tell a bad egg a secret, especially an embarrassing one, and it will only be kept if it shines a negative light on the bad egg as well.
It might be obvious that good and bad eggs are opposites, but why such folks are referred to as eggs at all isn’t so obvious. The terms a good egg and a bad egg make sense if they are considered as not only idiomatic expressions but as metaphors as well. Eggshells are just the surface of the egg, just as first impressions of people are largely based upon surface details.
It’s not until the egg is cracked that what’s inside is revealed. A very well-dressed, well-spoken person might turn out to be rotten on the inside or pure goodness all the way through. The first is a bad egg, and the other is a good one.