Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† mărăthrus or -os, i, m., or mără-thrum, i, n., = μάραθρον, fennel, called in pure Lat. feniculum, Plin. 8, 27, 41, § 99: marathros (v. l. marathrus), Ov. Med. Fac. 91.