Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ruptor, ōris, m. [rumpo], a breaker (not ante-Aug.); trop.: foederis, a violator, Liv. 4, 19 (with violator juris); 1, 28; 21, 40 fin.: 25, 31; Tac. H. 4, 57; Flor. 1, 3, 8: indutiarum, Liv. 8, 39: pacis, Tac. A. 2, 13.