Etymology

Almost all authorities derive this word straightforwardly from OE cnocian ‘to pound; strike, hit; knock (esp. at a door or gate)’. MED also suggests ON input, presumably because next to the far more common cnucian, spellings of OE cnoc- indicating /o/ are relatively rare, and restricted to late texts. A case can therefore be made for borrowing of or influence from the ON v. represented by OIcel knoka ‘to knock, thump’, which seems to continue a PGmc *knok- (cp. also MHG knochen) next to the *knuk- which lies behind OE cnucian. But it seems more likely that late OE cnoc- is simply a native ideophonic by-form of this sound-symbolic root. 

PGmc Ancestor

*knok- or *knuk-

Proposed ON Etymon (OIcel representative)

knoka ‘to knock, thump’
(ONP knoka (vb.))

Other Scandinavian Reflexes

Icel knoka, Norw knoka, ODan knoge, Dan knuge, Sw dial knoka, knåka

OE Cognate

cnocian ‘to pound; strike, hit; knock (esp. at a door or gate)’

Phonological and morphological markers

Summary category

CCC2a

Attestation

Apart from a few occurrences in 12c. mss of OE medical texts (where <cnuc-> is the dominant spelling), the v. is recorded by MED only with <o> in the stem; it is widespread from later ME, and attested no earlier than c1330(?a1300) Arth.& M.(Auch).

Occurrences in the Gersum Corpus

Gaw 414; Pe 727; Cl 1348; WA 639, 1599

Bibliography

MED knokken (v.) , OED knock (v.) , HTOED , Dance cnokez, de Vries knoka, Mag. knoka, Orel *knukôjanan, Kroonen *knuk(k)an, AEW cnocian, DOE cnucian; gecnucian; gecnucod