A person who bemoans; someone who frequently complains or expresses sorrow about something.
From 'bemoan' + the agent suffix '-er' (from Old English '-ere'). This suffix creates nouns meaning 'one who does X' or 'one who is X'.
The '-er' suffix for creating agent nouns is ancient and incredibly productive—we can turn almost any verb into a noun for a person (teacher, worker, runner), which is why English can describe professions and character types so easily.
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