The word "feather-brained" is used to describe someone who is foolish or lacking in intelligence. When looking for antonyms for this word, one can consider words that are associated with being wise, intelligent, or sensible. Words like astute, intelligent, shrewd, and brainy are all good antonyms for feather-brained. Other antonyms could include words like sagacious, wise, thoughtful, and analytical. These words are all associated with being thoughtful and intelligent, and they reflect a sense of deep thinking and introspection. In contrast, feather-brained represents someone who is flighty, inattentive, and not serious, so the antonyms represent people who take things seriously and are rational.