chump

Concise Oxford English Dictionary © 2008 Oxford University Press:
chump/tʃʌmp/

▶noun

• 1 informal a foolish person.

• 2 Brit. the thick end of something, especially a loin of lamb or mutton.

– phrases
off one's chump Brit. informal, dated mad.

– origin C18: prob. a blend of chunk1 and lump1 or stump.