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
Emmalyn	EM-ə-lin	English (Modern)
Epifanio	e-pee-FAH-nyo	Spanish, Italian
Herodion	-	Biblical, Biblical Latin, Biblical Greek, Ancient Greek
Zenobia	zə-NO-bee-ə (English)	Ancient Greek
Wenzeslaus	-	German
Vinh	-	Vietnamese