The expression "a curate's egg" originally meant something that is partly good and partly bad, but as a result is entirely spoiled. Modern usage has tended to change this to mean something having a mix of good and bad qualities.
The phrase derives from a cartoon in the humorous British magazine Punch on 9 November 1895. Drawn by George du Maurier and entitled "True Humility", it pictured a timid-looking curate taking breakfast in his bishop's house.[1]
The bishop says, "I'm afraid you've got a bad egg, Mr Jones." The curate replies, "Oh, no, my Lord, I assure you that parts of it are excellent!"
The expression refers to an objective understanding of the depicted scenario: since an egg that is even partly "bad" is effectively inedible, the supposedly "excellent" parts do not redeem it. The humour is derived from the fact that, given the social situation, the timid curate feels that he dare not complain about the quality of an inedible egg that would ordinarily be immediately rejected.
In the final issue of Punch (1992), the cartoon was re-printed with the caption: Curate: This f***ing egg's off!