off (one's) chump(redirected from off their chump)
off (one's) chump
Crazy or insane. He must be off his chump to pay that much money for that old car. Am I completely off my chump for starting a business in this economy?
off your chumpcrazy. British informal
The literal sense of chump meaning ‘a broad, thick block of wood’ led in the mid 19th century to its humorous use to mean ‘head’, with the implication of ‘blockhead’.