be off (one's) chump

To be crazy. In the UK, "chump" is a slang term for "head." Primarily heard in UK. You're off your chump if you think that plan will work.

Name	Pronounced	Usage
Yermolai	-	Russian
Ravindra	-	Hinduism, Indian, Hindi, Marathi, Telugu, Kannada
Suzanne	soo-ZAHN (French), soo-ZAN (English), suy-ZAHN-nə (Dutch)	French, English, Dutch
Annunziato	-	Italian (Rare)
Bikendi	-	Basque
Ecgberht	-	Anglo-Saxon