Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
immissĭo (inm-), ōnis, f. [immitto], a letting in, admission (rare but class.): fumi aut aquae, Dig. 8, 5, 8, § 5: sarmentorum ea aliorum amputatio, aliorum inmissio, an engrafting, Cic. de Sen. 15, 53.