One who causes discord or disagreement; a person who creates conflict or disrupts harmony.
From discord (verb) plus -er suffix, which creates agent nouns (people who do something). The pattern is similar to how 'teach' becomes 'teacher' or 'sing' becomes 'singer'.
This is an archaic or rare word—you won't find many people using 'discorder' to describe someone who causes trouble, probably because we have so many other words like troublemaker, agitator, or instigator!
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