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
Klim	-	Russian, Ukrainian
Romane	-	French
Cearbhall	-	Irish
Abner	['æbnə]
Chestirad	-	Medieval Slavic (Hypothetical)
Ejiroghene	-	Western African, Urhobo