Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. allapsus (adl-), a, um, Part. of allabor.
2. allapsus (adl-), ūs, m. [allabor], a gliding to, a silent or stealthy approach: serpentium, Hor. Epod. 1, 20: fontis, App. M. 5.