be off (one's) chump

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be off (one's) chump

To be crazy. In the UK, "chump" is a slang term for "head." You're off your chump if you think that plan will work. You can't tell people you want to start a business selling bees as pets—they'll think you're off your chump! There's a guy on our corner who shouts about the end of the world—pretty sure he's off his chump.