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The verb is derived from Late Middle English knappen (“to strike (something)”); further etymology uncertain, probably related to Middle Dutch knappen, cnappen (“to break (something) with a sharp sound, snap; to crack or crackle”) (modern Dutch knappen), probably ultimately onomatopoeic. The noun is derived from Late Middle English knap, knappe (“sharp blow, strike”); further etymology uncertain, possibly from the verb (see above), or related to an ancestor of Danish knep and Swedish knäpp (“a flick, rap, snap”), probably ultimately onomatopoeic.

From Middle English knap, knappe (“small projection, knob (button, tassel, tuft, etc.); hill; hilltop; etc.”), from Old English cnæp, cnæpp (“summit, top”); further etymology uncertain, possibly related to Old Norse knappr (“small projection, knob (button, head of a stick, etc.)”) (whence English knop), from Proto-Germanic *knappô, *knuppô.

Origin uncertain; possibly: * from Middle English *kneppen (compare Middle English knippette (“pincers for cracking nuts”)), from Old Norse kneppa (“to pinch, press, squeeze; to button, clasp; to hug”), from Proto-Germanic *knappijaną (“to clamp; to squeeze”), from Proto-Indo-European *gnebʰ- (“to constrict, tighten; to press”); * related to knap (etymology 1); or * imitative of a mouth snapping shut; compare gnap (“to snap at”) (obsolete except Scotland), nab (“to bite gently, nibble”) (obsolete except Southern and Western England).

See nap (etymology 5).