Harebrain is a slang term that refers to someone who is scatterbrained, impulsive, or foolish. Antonyms for harebrain include words like intelligent, clever, astute, brilliant, insightful, perceptive, and wise. These words describe someone who is capable of making rational decisions, displaying foresight, and solving problems. Other antonyms for harebrain include words such as focused, organized, methodical, and disciplined. These are all qualities that are associated with someone who has an ordered mind and can stay on task. Using these antonyms is a useful way to describe someone who is the opposite of a harebrain, and who has qualities that are desirable in a variety of situations.