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
Jamesina	-	Scottish
Zehra	-	Turkish
Crispinus	-	Ancient Roman
Belenus	-	Celtic Mythology
Thucydides	thoo-SID-ə-deez (English)	Ancient Greek (Latinized)
Rashed	-	Arabic