Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. illapsus (inl-), a, um, Part., from illabor.
2. illapsus (inl-), ūs, m. [illabor], a falling, gliding, or flowing in: umoris illapsus atque exitus, Col. 2, 2, 11: serpentino illapsu, Ambros. Ep. 6, 42: gregis illapsu fremebundo territus, Sil. 3, 463.