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
Susumu	soo-soo-moo	Japanese
Pankratios	-	Ancient Greek
Erling	ER-ling	Swedish, Norwegian, Danish
Rilla	RIL-ə	English
Alexandrie	a-lek-sawn-DREE	French (Rare)
Eutychia	-	Ancient Greek