Hi 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
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dave_anon Use your loafThat's originally Cockney rhyming slang - loaf - loaf of bread - head.