Etymology

cp. OIcel (í) gegn ‘against, opposite, contrary to, in turn’; cp. OE on-gegn, on-gēan ‘against’; cp. OS an-gegin, OHG in-gagin. Old Norse input is suggested by lack of palatalization /ɡ/. ME -gayn is explicable phonologically as a direct derivation of ON gegn; but given the vocalism of the first syllable the whole word is often treated as a variant of ME aʒayn (< OE on-gegn) with substitution of /ɡ/ for /j/ (thus EVG: 100 and Onions 1953: 111).  See also gayn (etc.), gayne.

PGmc Ancestor

*gagni-

Proposed ON Etymon (OIcel representative)

(í) gegn 'in opposition, against, in return' 
(ONP gegn (1) (sb.))

Other Scandinavian Reflexes

Far ígen, Icel gegn,  Dan igen, OSw gen

OE Cognate

on-gegn, on-gēan  'against'

Phonological and morphological markers

absence of palatalization of */ɡ/

Summary category

A1*c

Attestation

Spellings reliably representing /ɡ/ are rare in the South and West until  c1400, but are normal in the Chaucer citations in MED and become increasingly frequent in southern texts in the fifteenth century; see also LALME dot map 220.

Occurrences in the Gersum Corpus

Gaw 384, 1638; Pe 28, 326, 1199, etc.; Cl 266, 826; WA 50, 82, 188, etc.

Used as a prefix in Cl 931 agayn-tote 'backward glance'. WA 636 agayn stande 'resist' is printed as one word by TPD.
 

Bibliography

MED ayēn (adv.) , OED again (adv., prep. and conj.) , ODEE again, HTOED , EDD again (conj. and adv.), Dance agayn,  Bj. 151, de Vries gegn, Mag. gegn (2), Orel *ʒaʒnē, AEW gegn (2)