A person who is morally bad or behaves poorly; British slang for a scoundrel or worthless person.
From 'rot' (to decay or deteriorate) + '-er' (one who). The word emerged in British English around the 1890s, comparing a bad person to something that is rotting or falling apart. It was popularized in public school literature.
A 'rotter' in British slang is more playful than serious — you might call someone a rotter if they stole your dessert or stood you up, not if they committed a terrible crime! It's the kind of word Roald Dahl characters would use.
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