What's the "een" in Halloween?
Old English fenn "mud, mire, dirt; fen, marsh, moor," from Proto-Germanic *fanjam- (cf. 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" (cf. 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.
type of bog (q.v.), especially a low-lying area, wholly or partly covered with water and dominated by grasslike plants, grasses, sedges, and reeds. In strict usage, a fen denotes an area in which the soil is organic (peaty) and alkaline rather than acid.