One who or that which distempers; something that causes disorder or irritation.
From distemper + -er (suffix forming agent nouns). The -er suffix has Old English origins and creates nouns for one who performs an action.
The -er suffix is incredibly useful in English—it turns almost any verb into a noun for 'someone who does this,' which is why we have countless words like baker, maker, teacher, and in this case, someone who disturbs the balance.
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