One who besets; a person or thing that persistently troubles, harasses, or surrounds another.
From 'beset' + '-er' (agent noun suffix indicating one who performs an action). The '-er' suffix transforms verbs into nouns naming the actor, an extremely productive pattern in English.
Agent nouns with '-er' are so common that English speakers create them instinctively—'teach' becomes 'teacher,' 'run' becomes 'runner,' 'beset' becomes 'besetter'—showing how the suffix is alive and productive in modern English!
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