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
Cynthia	SIN-thee-ə (English)	English, Greek Mythology (Latinized)
Idan	-	Hebrew
Magnolia	[mæg'nəuliə]
Beverley	BEV-ər-lee	English
Abbey	['æbi]
Yuuto	yoo:-to	Japanese