From Middle English fen, fenne, from Old English fenn, Proto-West Germanic *fani, from Proto-Germanic *fanją (compare West Frisian fean, Dutch veen, German Fenn, Norwegian fen), from Proto-Indo-European *pen-. Compare Middle Irish en, enach, Old Prussian pannean, Sanskrit पङ्क.
From fan, by analogy with men as the plural of man.