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
Samo	-	Slovene, Medieval Slavic
Shirley	['ʃə:li]
Aparajita	-	Bengali, Indian, Hindi
ClÍOdhna	KLEE-u-na	Irish, Irish Mythology
Xue	-	Chinese
Kermit	['kɜ:mit]