dumbbell

Concise Oxford English Dictionary © 2008 Oxford University Press:
dumb-bell

▶noun

• 1 a short bar with a weight at each end, used typically in pairs for exercise or muscle-building.

• 2 N. Amer. informal a stupid person.

– origin C18: orig. denoting an object similar to that used to ring a church bell (but without the bell, so ‘dumb’).