Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
spissĭtas, ātis, f. [spissus], compactness, density (post-Aug. and very rare): quercus (opp. raritas), Vitr. 2, 9 med.: tritici, Plin. 18, 30, 73, § 304.