Superlative form of fell; most fierce, cruel, or destructive; the most terrifying or severe.
From Old English fel (cruel, fierce) plus the superlative suffix -est; fell as an adjective is now archaic but survives in phrases like 'fell purpose' or 'a fell deed'.
The adjective 'fell' is so archaic that few modern speakers recognize fellest—yet Tolkien and other fantasy authors revived it because it sounds fittingly dark and old, proving that literary convention can resurrect forgotten words.
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