Etymology

cp. OIcel ey, ei ‘always’ < the acc. sg. of PGmc *aiwaz ‘age, time’; cp. OE ā, Go áiw, OFris ā, OS ēo, OHG io. See also ayquere. 

PGmc Ancestor

the acc. sg. of *aiwaz

Proposed ON Etymon (OIcel representative)

ey, ei 'always, eternally, forever'
(ONP ei (2) (adv.), ey (4) (adv.))

Other Scandinavian Reflexes

Icel ei, e

OE Cognate

ā 'always, ever'

Phonological and morphological markers

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

Summary category

A1*c

Attestation

Common since Orrm; widespread in later ME, though apparently more restricted earlier on; MnE dial in N. 

Occurrences in the Gersum Corpus

Gaw 26, 73, 128 etc.; Pe 44, 56, 101 etc.; Cl 114, 117, 132 etc.; Pat 8, 90, 130 etc.; Erk 278; WA 239, 242, 265, etc.

See the remarks in McGee 488-9 on its replacement in some mss of SJ and of Chaucer’s works.

Bibliography

MED ai (adv.) , OED ay, aye (adv.) , HTOED , EDD aye (adv. 1), Dance ay, Bj. 40 (64), de Vries ei (1), Mag. ei (1), ey (3), Bj-L. æve, Orel *aiwaz ~ *aiwiz, AEW ā (2)