A "curate's egg" is a phrase used to describe something that is at least partly bad, but with some arguably redeeming features.
In its original context, the term refers to something which is obviously and essentially bad but which is wilfully described euphemistically as being only partly so, its supposed good features being credited with undue redeeming power. Its modern usage varies, with some authorities defining it as something that is an indeterminate mix of good and bad and others stating it implies a preponderance of bad qualities.
The term derives from a cartoon published in the humorous British magazine Punch on 9 November 1895. Drawn by George du Maurier and entitled "True Humility", it pictures a timid-looking curate taking breakfast in his bishop's house. The bishop remarks with candid honesty to his lowly guest: "I'm afraid you've got a bad egg, Mr Jones." The curate replies, desperate not to offend his eminent host and ultimate employer: "Oh, no, my Lord, I assure you that parts of it are excellent!"
The term relies on an objective analysis and intuitive understanding of the depicted scenario: a self-contained egg cannot be both partially spoiled and partially unspoiled. To pretend to find elements of freshness in a bad egg is thus a desperate attempt to find good in something which is irredeemably bad. The humour is derived from the fact that, given the social situation, the timid curate is so obsessively fearful of giving offence that he cannot even agree with his superior's acknowledgement that he has served a bad egg and thereby ends up looking absurd himself by exposing his obsequiousness.
In the final issue of Punch published in 1992, the cartoon was re-printed with the caption: Curate: This f***ing egg's off! The contrast is thus drawn with the modern era where people are implied to have little care for the niceties of Victorian over-stretched good manners towards what were then termed social superiors.
- "The past spring and summer season has seen much fluctuation. Like the curate's egg, it has been excellent in parts." (Minister's Gazette of Fashion, 1905)
- "All the same it is a curate's egg of a book. While the whole may be somewhat stale and addled, it would be unfair not to acknowledge the merits of some of its parts." (Oxford Magazine, 1962). 
- "Like the curate's egg, the details of Wegener's hypothesis were good in parts." (John Gribbin, The Scientists, 2001).
- "Fishman... attempted a 'systematization' of SWH [Sapir-Whorf Hypothesis], suggesting that its extreme versions were untenable..., but that, like the curate's egg, it was excellent in parts." (Philip K. Bock, "World view and language", in Encyclopedia of Linguistics, ed. William Bright, vol. 4, Oxford University Press, 1992, p. 250).