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
Kresimir	-	Medieval Slavic
Loviisa	LO-vee:-sah	Finnish
Luned	LIN-ed (Welsh)	Welsh, Welsh Mythology, Arthurian Romance
Jacklyn	JAK-lin	English
Jose	[dʒəuz]
Zawisza	-	Polish (Archaic)