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
Maris	MER-is, MAR-is	English (Rare)
Astor	AS-tər	English (Rare)
Minos	-	Greek Mythology
Avram	-	Hebrew, Biblical Hebrew
EguzkiÑE	-	Basque
Turid	-	Swedish, Norwegian, Danish