More beast-like, savage, or cruel in behavior or appearance; comparative form of 'beastly.'
From 'beastly' (Middle English from Old French 'beste') plus the comparative suffix '-er', following standard English patterns for comparing adjectives.
The word 'beastly' was especially popular in Victorian English as a mild insult meaning 'unpleasant,' so a Victorian might call bad weather 'beastly'—a funny example of how languages soften harsh words over time.
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