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
Oenone	-	Greek Mythology (Latinized)
Andreina	-	Italian
Berker	-	Turkish
Alexander	a-lig-ZAN-dər (English), ah-lek-SAHN-der (German), ah-lək-SAHN-dər (Dutch), AW-lek-sawn-der (Hungarian)	English, German, Dutch, Swedish, Norwegian, Danish, Icelandic, Hungarian, Slovak, Biblical, Ancient
Hertha	HER-tah	German
Claudio	KLOW-dyo	Italian, Spanish