Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ărāter, tri, m. (a rare form for aratrum), a plough, Hyg. Limit. p. 204 Goes.
ărātrum, i, n. [ἄροτρον], a plough (the inventor of which was Byzyges, acc. to Plin. 7, 56, 57, § 199; or Triptolemus, acc. to Verg. G. 1, 19. The parts of it were temo, stiva, manicula, vomer, buris, aures, and dentale. For a description of it, v. Verg. G. 1, 162 sqq.; Pauly’s Real-Ency. I. pp. 665 sq.; and Smith, Dict. Antiq.); Lucr. 1, 313; 5, 219: curvi moderator aratri, id. 5, 933, and id. 6, 1251; Cic. Rosc. Am. 18; id. Agr. 2, 25; id. N. D. 2, 63, 159; Verg. G. 1, 19; 1, 170 et saep.: imprimere aratrum muris, to press the plough into the walls (of a town), i. e. to turn a town into arable land, to destroy completely, Hor. C. 1, 16, 20; cf. Sen. Clem. 1, 26, 4; used for marking the boundaries of new towns, Cic. Phil. 2, 40: Aeneas urbem designat aratro, Verg. A. 5, 755, ubi v. Serv.; Cato ap. Isid. Orig. 15, 2; Inscr. Orell. 3683.