A person who frets; someone who worries frequently or excessively about things.
From 'fret' + the agent suffix '-er', creating a noun for one who performs the action of fretting, following standard English word-formation rules.
Every language needs a word for 'someone who does X too much'—English's '-er' suffix is incredibly productive, which is why we can coin 'fretter' even if it's not in every dictionary.
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