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
Anoush	-	Armenian
Iustinus	-	Late Roman
Hyginus	-	Ancient Greek (Latinized)
Ashlynn	ASH-lin	English (Modern)
Alban	['ɔ:lbən]
LÉOnne	-	French