Etymology

From ON, either directly < an ON v. *weikna represented by Norw dial veikna, OSw vikna, MSw, Sw vekna (OED, MED) or formed within English on wayke (adj.) (cp. waykis (v.)).

PGmc Ancestor

*weik(w)a-

Proposed ON Etymon (OIcel representative)

veikr (veykr) (adj.) ‘weak’ 
(ONP veikr (adj.))

Other Scandinavian Reflexes

Far veikur, Icel veikur, Norw veik, Dan veg, Sw vek; Norw dial veikna, OSw vikna, MSw vekna, Sw vekna

OE Cognate

wāc (adj.) 'weak'; cp. wācian 'to become weak, languish'

Phonological and morphological markers

ON /ei/ &lt; PGmc */ai/

Summary category

A1*c

Attestation

hapax legomenon (though the adj. is widespread from the 14c. onwards)

Occurrences in the Gersum Corpus

Cl 1422

Bibliography

MED weiknen (v.) , OED weaken (v.) , HTOED ; see further wayke (adj.)