A person who digresses; someone who tends to wander away from the main topic in conversation or writing.
From 'digress' (verb) + '-er' (agent noun suffix). This term creates a noun describing someone characterized by the habit of digressing, rarely used in modern English.
A 'digresser' is basically a professional tangent-taker—and interestingly, many great storytellers and comedians are natural digressers, using side comments to build unexpected humor.
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