血气 literally means 'blood and qi/energy', but as a noun it refers to a person's temperament, vigor, or spiritedness — especially the kind of youthful passion or hot-blooded courage that drives action. It often appears in phrases like 血气方刚 (full of vigor and vitality, said of young people) and 血气之勇 (courage born of raw emotion, sometimes implying recklessness). The word carries a slightly classical or literary register; you'll encounter it in formal writing, historical narratives, and discussions of character.