A person or thing that displeases or causes displeasure.
From displease plus the agent noun suffix -er, which creates nouns for people or things that perform an action. Common in older English, now rarely used since 'annoyance' or 'nuisance' convey the meaning better.
The '-er' suffix is one of English's oldest tools for naming people—'teach-er,' 'sing-er,' 'displeas-er'—yet we've mostly abandoned 'displeaser' in favor of other words, showing how some formations are productive while others fade away.
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