Lewis & Short

lăpăthum, i, n., and lăpăthus, i, f. (m., Lucil.; v. infra. Ante-class. collat. form lăpăthĭum, ii, Varr. ap. Non. 550, 17; id. L. L. 5, § 103 Müll.), n., = λάπαθον or λάπαθος, sorrel.
Form lapathum, Plin. 20, 21, 85, § 231.
Form lapathus, Col. poët. 10, 373.
In masc., Lucil. ap. Cic. Fin. 2, 8, 24.
In a dub. form: lapathi brevis herba, Hor. S. 2, 4, 29; id. Epod. 2, 57.