Having a fever, or behaving with great excitement and intensity like someone who is sick with fever.
From Old English 'feofor' (fever), combined with '-ish' (resembling or characterized by). The word evolved to describe both physical fever and fever-like excited behavior.
When you have a fever and get delirious, you act with intense energy and confusion—so 'feverish activity' doesn't mean you have a fever, it means you're acting as confused and intense as if you did!
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