Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
dissĭmŭlātor, ōris, m. [dissimulo], a dissembler, concealer, Sall. C. 5, 4; Quint. 2, 2, 5; Tac. H. 2, 56; Hor. Ep. 1, 9, 9 al.