One who or that which disquiets; something that causes uneasiness or anxiety.
From disquiet (verb) + -er (agent suffix). The -er suffix creates nouns for people or things that perform an action.
A 'disquieter' is something or someone that disturbs your peace—the word is rare, but it elegantly captures the concept of whatever keeps you restless, whether it's an annoying person or an unsettling thought.
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