Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
cŏassātio (cŏax-), ōnis, f. [coasso], a joining of timber (boards or planks) together; hence in concr., a boarded floor, Vitr. 6, 3, 9; 7, 1, 1 sq.
In plur., Plin. 36, 25, 62, § 186.