Where does “fenu” come from?

fenu (Czech) comes from Czech fena, from German Fenn, from Middle Low German venne, from Old Saxon fenni, from Proto-West Germanic *fani, from Proto-Germanic fanją, from Proto-Indo-European pen- — bog, mire; moist; wet; mud; swamp; water; mire,...

fenu (Czech): dative singular of fena; genitive/dative/locative...