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British slang of unknown origin meaning "crazy", like "off your rocker" or "out of your mind".
"I can't believe she tried to do that with only duct tape, a banana, and a bottle of Windex... that bitch was off her chump!"

"That may be true, but you're off your chump if you think you can get off without putting 'off your chump' in the example usage here."
by offmychump December 15, 2008