Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
impŏsĭtĭo (inp-), ōnis, f. [impono], gramm. t. t., the application of a name to a thing, Varr. L. L. 8, § 5 Müll.; 10, § 51; § 61 (in Plin. 27, 13, 115, § 141, the true reading is in potione).