From Middle High German knîp, knîf, from Proto-Germanic *knībaz. The expected Luxembourgish form is *Knäif; such forms with -f do exist in some dialects of Central Franconian. The -p is possibly due to conflation with Central Franconian kneipe, kniepe (“to squeeze, to nip”), from Middle Low German knīpen, though Luxembourgish knäipen now only means “to copy another pupil’s test” (derived from “to screw up one’s eyes”, compare German kniepen). Alternatively, the -p could be explained as Rhine Franconian.