adjective
Characteristic of or resembling a fen (“characteristically alkaline wetland containing peat below the waterline”); marshy, swampy; also, of land: containing a fen or fens.
Now chiefly of plants: growing or living in a fen.
(obsolete, also figurative) Muddy; hence, dirty, filthy.
noun
(horticulture, colloquial) Alternative form of fennie (“fenestration”).