The term "something brain" can be interpreted in different ways, but if we take it to mean a brain that is active, engaged, and productive, the antonyms would be sluggish, idle, and unproductive. A sluggish brain is slow and not responsive, while an idle brain is inactive and not doing anything. An unproductive brain is one that's not generating any new ideas or solving problems. Additionally, a distracted brain, a preoccupied brain, and a scattered brain are antonyms of something brain as they all refer to brains that lack focus, concentration, and clarity. In contrast, a sharp brain, an agile brain, and a vigorous brain are synonyms of something brain as they all describe brains that are quick, active, and alert.