Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
fastīgātĭo, ōnis, f. [fastigo], a sharpening to a point, pointing (post-Aug. and very rare): ut fastigatio laevi descendat cuneo, Plin. 17, 14, 24, § 106: recta, App. Flor. 2, 364.