The phrase "crack brains" refers to someone who is intellectual or highly intelligent. However, there are several antonyms for this phrase that describe people who lack intelligence or critical thinking skills. These antonyms include the terms "dim-witted," "stupid," "ignorant," and "unintelligent." Those who are "brain-dead," "obtuse," or "slow-witted" may also be antonyms for someone who is referred to as "crack brain." Using these antonyms in context can help to create a clearer picture of the intelligence level of an individual or group and can be useful in discriminating between those who are highly intelligent and those who are not.