What does "a curate's egg" mean — and why is it funny?

informal, British

Meaning

Something that is partly good and partly bad, yet politely described as if the good parts make up for the rest.

Where it comes from

From a famous 1895 Punch cartoon, in which a nervous young curate, served a bad egg by his bishop, insists that 'parts of it are excellent'.

Why it is funny

The humor is the desperate politeness. A rotten egg cannot really be good in parts, and the phrase preserves forever that moment of someone too timid to admit the obvious.

Used in a sentence

"The new restaurant was a curate's egg — lovely starters, dreadful mains."