A British idiom which is often mistakenly thought to describe something which is partly good and partly bad. Its real meaning is somewhat harder to define though, and is best explained by describing the circumstances of its origin.
In 1895 the British humorous magazine Punch published a George du Maurier cartoon entitled "True Humility" which depicted a curate dining at his bishop's house. The bishop remarks that his guest has unfortunately been served a with a bad egg. The curate, desperate to remain polite and not give offence to his superior, replies, "Oh, no, my Lord, I assure you that parts of it are excellent!".
Of course, there are no good parts in a bad egg; once an egg goes bad, the whole thing is rotten. The cartoon was actually making a sly comment about deference and ambition, and perhaps also poking a little fun at the extremely repressed nature of British polite society at that time.
While this is probably not many people's idea of a funny joke now, it was apparently perfect for the end of the 19th century. The line "parts of it are excellent" caught on quickly and became a hugely popular catchphrase, gaining usage in the media and spreading through all levels of society. It was one of the strongest memes of its day, and because of it the term 'curate's egg' entered the language.
So, my somewhat clumsy definition of a curate's egg would be: something which you pretend is a mixture of good and bad, but which in reality you know perfectly well is nothing but rotten.