Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Ălĭacmon, v. Haliacmon.
Hălĭacmon (Aliacmon), ŏnis, m., a stream forming the boundary between Macedonia and Thessaly, now Vistritza, Plin. 4, 10, 17, § 34; Caes. B. C. 3, 37; Liv. 42, 53, 5; Claud. B. G. 179.