To deliberately and unlawfully kill another person.
From Old English 'morþor' and Germanic roots related to 'mort' meaning death. The word came to mean specifically an unlawful, intentional killing, distinguishing it from accidental death or lawful execution.
The word 'murder' has been used for a thousand years, but it specifically means the *unlawful* kind of killing—which is why soldiers in a war aren't technically 'murderers' and why 'justified' killings aren't called murder, because the law is what makes it count.
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