Grisping is another word for twilight, either at the break or end of day.
That’s a vanishingly rare word, found in only a handful of literary records dating from the late 1500s, before being resurrected in nineteenth century literature and glossaries and ultimately listed in the Oxford English Dictionary.
This also appears to be just one of a number of variations on a theme here: other dictionaries include entries for the likes of grapslin, grasplin, gropsing and grasping, and even the word grasp itself is listed as a synonym for twilight in the OED.
It’s likely that grasp is the origin of all these forms, presumably in the sense of twilight being the last ‘grasp’ of nighttime or daytime. Etymologically, grasp is a Middle English development of an older (though unattested) Old English word græpsen, which shares the same ultimate Germanic origins as words like grip and grope.
This Old English form seems to have undergone metathesis (i.e. a reordering of its sounds and letters) in Middle English, which not only left us with the –sp spelling of grasp we still have today, but likely accounts for the variation among the –ps– and –sp– spellings of all those grapslins and gropsings listed above.