A person who commits murder; someone who deliberately and unlawfully kills another person.
From 'murder' plus the '-er' suffix meaning one who does something. The suffix '-er' comes from Old English and Germanic roots and is used to create agent nouns.
When you add '-er' to any verb in English, you make a noun for the person doing that action—'teach' becomes 'teacher,' 'bake' becomes 'baker,' and 'murder' becomes 'murderer,' showing how productively English creates new words from old roots.
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