fen (n.) Look up fen at Dictionary.com
"low land covered wholly or partly by water," Old English fenn "mud, mire, dirt; fen, marsh, moor," from Proto-Germanic *fanjam- "swamp, marsh" (cognates: Old Saxon feni, Old Frisian fenne, Middle Dutch venne, Dutch veen, Old High German fenna, German Fenn "marsh," Old Norse fen, Gothic fani "mud"), from PIE *pen- "swamp" (cognates: Gaulish anam "water," Sanskrit pankah "bog, marsh, mud," Old Prussian pannean "swampland"). Italian and Spanish fango, Old French fanc, French fange "mud" are loan-words from Germanic. The native Latin word was limus or lutum.