Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
plăcĭdo, āvi, āre, v. a. [placidus], to soothe, calm (late Lat.): cursus suos, Ambros. Virg. 3; id. de Cain et Abel, 1, 3, 11; id. Jac. et Vit. Beat. 2, 6, 28.