Early 16th century; earliest use found in William Dunbar (?1460–?1530), poet and courtier. Probably the reflex of a borrowing from early Scandinavian (compare Norwegian regional knul, Old Swedish knula (Swedish (now regional) knula), Danish regional (Bornholm) knul, and (with different stem vowel) Old Swedish knyla (Swedish knyla (now regional), †knyl, †knøla, knøl (now the usual form)), early modern Danish knyl, all in sense ‘lump, knob, protuberance, excrescence’) from an extended form of the same Germanic base as Old Icelandic knúi knuckle; other extended forms of the base are reflected in knob, knoll, knop, knot, knur, etc.; further etymology uncertain. In later use perhaps also from German regional (Low German) Knull, Knulle lump, clod, swelling, excrescence.