a good egg
informal, colloquial phrase

- a likeable person

Merriam-Webster Dictionary

- someone who you can depend on to be honest and kind

Longman Dictionary

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ORIGIN

A good egg is a friendly, old-fashioned way to talk about a good, a kind, or a likeable person. Someone who you're very fond of.

The expression originally came from its opposite, bad egg which is British public school slang from the 1800's for someone who behaves in a bad or dishonest way. Fifty years later, good egg emerged as a casual way to talk about a good chap or a decent fellow, ladies also included :-)

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SYNONYMS

a jolly good fellow, a fine chap, a good old brick, (s)he's rare fun

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SOME EXTRA EGGY PHRASES

- to have egg on one's face = to look foolish or be embarrassed

- kill the goose that lays the golden egg = to destroy something that is profitable

- a nest egg = an amount of money that one has saved

- put all your eggs in one basket = to risk losing everything by putting  all your efforts or all your money into one plan or one course of action

- you can't make an omelette without breaking eggs = something you say when it is difficult to achieve something important without causing any unpleasant effects