Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Ănī̆grŏs, i, m., = Ἄνιγρος, a little river in Elis, rising on Mount Lapithus, now Mauropotamo; its waters were muddy and of an unpleasant odor, Ov. M. 15, 282; cf. Mann. Greece, p. 519.