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
Fleur	FLUUR (French, Dutch), FLUR (English)	French, Dutch, English (Rare)
Antonis	-	Greek
Sanford	['sænfəd]
Riaz	-	Arabic
ÁLmos	AHL-mosh	Hungarian
Suhail	-	Arabic, Urdu