Etymology

When it is used as a rel. pron. or conj., ME at is usually explained as a direct borrowing of ON at (cp. OIcel at), < PGmc *þat with loss of initial consonant; given the Northern distribution of ME and MnE at in this sense, some ON input seems probable.  However, it is difficult to rule out a parallel native development of the native cognate OE ðæt, which is very plausible in the sequence þat þat (> þat tat > þat at, by assimilation and haplology) (see SPS 415).  This is how most recent commentators have understood the instance at Pe 536 (‘and wyrkeʒ and dotʒ þat at ʒe moun’), beginning with Gordon and Onions (1932: 135, citing Sisam; see also EVG 536n and McGee); and in some dialects (such as that of WA) the variant at may have spread from such contexts and attained wider currency as a rel., with or without support from ON at.  Most authorities entertain a mixed origin (thus OED, Bj., MED).

PGmc Ancestor

*þat

Proposed ON Etymon (OIcel representative)

at ‘who, which, that’
(ONP at (conjunct.))

Other Scandinavian Reflexes

Far at, Icel að, Norw at, Dan at, OSw at, Sw att

OE Cognate

ðæt (conj. and adv.) ‘so that, in order that, after that, then, thence’

Phonological and morphological markers

Summary category

CC2c

Attestation

Following three occurrences of OE æt as a conj. in late Nhb (see DOE s.v. æd, æt; SPS 415), ME at (conj., rel. pron.) is attested in N dial from the early 13c onwards, surviving into MnE (OED, EDD).  The phrase þat at (that at) appears in a handful of MED's citations (s.v. that rel. pron.), mainly N/EM (but notice also e.g. c1450(c1415) Roy.Serm.(Roy 18.B.23)).

Occurrences in the Gersum Corpus

Pe 536; WA 10, 56, 68 etc.

Vant takes issue with Gordon’s argument against interpreting Pe 536 at as (1) (because at ‘does not occur as a relative elsewhere in this group of poems’, EVG 536n) by reading such an instance at Gaw 2205, where he maintains MS at (cp. emendations in TGD etc. and see Onions and Gordon 1933: 179).

Bibliography

MED that (rel. pron.), OED at (conj. and pron.) , Bj. 201, de Vries at (4), Mag. að (4), Orel *þat, AEW ðæt