Someone feebleminded is unintelligent. A feebleminded person lacks smarts and does dumb stuff like bring a pit bull to a cat show or drive a convertible in a snow storm. A feebleminded person has a feeble, or weak, mind.
To be feebleminded is to be mentally slow. People with an IQ below a certain number are feebleminded in a technical sense. But this word is used more often for people who act dumb without the benefit of a medical excuse. Riding a bike without a helmet is feebleminded. Challenging a cheetah to a running race is feebleminded. To be feebleminded is to act like an idiot.