Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
vervactum, i, n. [vervago], fallow ground, a fallow field, Cato, R. R. 27; Varr. R. R. 1, 44, 2; Col. 11, 2, 32; 11, 2, 52; Plin. 18, 19, 49, § 176; Pall. 4, 2.