A person or thing that kills or destroys something. In informal usage, something extremely impressive or effective.
From Middle English, from the verb 'kill' plus the agent suffix '-er'. The word 'kill' comes from Middle English 'killen', possibly from Old English 'cwellan' meaning 'to kill, murder, execute'.
The word 'killer' underwent a fascinating semantic shift in the 20th century, developing positive slang meanings like 'killer app' in technology or 'that's killer!' meaning 'that's awesome!' This transformation from purely negative to potentially positive reflects how language adapts to express admiration for something's power or effectiveness.
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