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
Knut	KNOOT (German)	Swedish, Norwegian, Danish, German
Roe	[rəu]
Cheftzi-Bah	-	Biblical Hebrew
Francesca	frahn-CHES-kah (Italian)	Italian, Catalan
Marama	-	Polynesian Mythology
Krysia	KRI-shah	Polish