Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
impostor (inp-), ōris, m. [impono, II. B. 3.], a deceiver, impostor (post-class.; cf.: planus, sycophanta), impostores aut mendaces aut litigiosi, Dig. 21, 1, 4, § 3; Hier. Ep. 38 fin.; 54, 5; Paul. Nol. Carm. 21, 338.