“But gone here,” he said. “I’ve thought so for some time. Poor old chap, so that’s why he had to give up and come down here. In the family, very likely. He’s got a nephew who’s quite off his crumpet.”

Good question. In slang, the human head is referred to in various ways. Here, it is being likened to a kind of small cake

- He's off his crumpet
- He's off his head
[= mad]

On the web, I've found this attributed to Australian slang, but I would also say that the expression was popular in London, but quite a long time ago

dave_anon Use your loaf
That's originally Cockney rhyming slang - loaf - loaf of bread - head.
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I was desperately trying to think of another quite old idiom where bakery is involved - it just occurred to me:

- Use your loaf

- Use your common sense

Dave :-)
 fivejedjon's reply was promoted to an answer.
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Aha! Got it

In American English: (Most Americans don't know what crumpets are.)

He’s got a nephew who’s quite off his rocker.
This one is actually from Agatha Christie's Murder of Roger Ackroyd.
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