Etymology

Both OE and ON had a wk. causative v. formed on PGmc *wēpnan (cp. OIcel vápn, wǣpen 'weapon', OFris wēpin, wēpen, OS wāpan, OHG wāfan), cp. OE wǣpnian 'to arm', OIcel vápna 'to furnish with arms' (and further cp. OHG wāfenen). Just as with wapen (n.), forms of the v. with variant vocalism in ā or ō from N texts are explained by most authorities as a reflex of the ON cognate. 

PGmc Ancestor

*wēpn-

Proposed ON Etymon (OIcel representative)

vápna 'to furnish with arms'
(ONP vápna (vb.))

Other Scandinavian Reflexes

Far vápna, Icel vopna, Norw våpna, Sw våpna

OE Cognate

wǣpnian 'to arm'

Phonological and morphological markers

ON /ɑ:/ < PGmc */e:/ (1)

Summary category

A1*c

Attestation

Orthographic variants indicating ā or ō are attested as early as c1275(?a1200) Lay.Brut (Clg A.9) and a1325(c1250) Gen.& Ex. (Corp-C 444), and characteristic of N texts (see MED, Bj.).

Occurrences in the Gersum Corpus

WA 1250

Bibliography

MED wē̆pnen (v.) , OED weaponed (adj.) , HTOED , Bj. 84, 93, Mag. vopn, AEW wǣpnan