Webster's Revised Unabridged Dictionary · 1913

Fen

Fen (fen) , noun

[Anglo-Saxon fen, fenn, marsh, mud, dirt; akin to Dutch veen, OFries. fenne, fene, Old High German fenna, German fenn, Icelandic fen, Gothic fani mud.]

Low land overflowed, or covered wholly or partially with water, but producing sedge, coarse grasses, or other aquatic plants; boggy land; moor; marsh.
'Mid reedy fens wide spread. — Wordsworth

Fen is used adjectively with the sense of belonging to, or of the nature of, a fen or fens.

Collocations (5)
Fen boat , a boat of light draught used in marshes.
Fen duck (Zoology) , a wild duck inhabiting fens; the shoveler. [Provincial English]
Fen fowl (Zoology) , any water fowl that frequent fens.
Fen goose (Zoology) , the graylag goose of Europe. [Provincial English]
Fen land , swamp land.