A person who produces cacophonies (harsh sounds), or ironically, someone known for making unpleasant or discordant music.
From 'cacophony' + the agent suffix '-ist' (one who does); parallel to 'symphonist' for someone who creates symphonies.
This word is rarely used seriously—it's mostly employed humorously to roast bad musicians, but the '-ist' suffix shows how productively English can create agent nouns from any noun describing an activity or art form.
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