A curate's egg

The expression "a curate's egg" originally meant something that is partly good and partly bad, but as a result is entirely spoilt. Modern usage has tended to change this to mean something having a mix of good and bad qualities; an example in conversation would be, "Ah Tisshaw, how was your honeymoon?" "Something of a curate's egg, I'm afraid; the hotel was top-notch, but the sex was awful."
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Meaning
Something bad that is called good out of politeness or timidity..
Origin
The origin of the phrase, the George du Maurier cartoon - "True Humility", printed in the magazine Punch, 9th November 1895, gives fuller insight into its meaning, which relies to some extent on an appreciation of irony.

TRUE HUMILITY.
Right Reverend Host. “I’m afraid you’ve got a bad Egg, Mr. Jones!”
The Curate. “Oh no, my Lord, I assure you! Parts of it are excellect!