Etymology

cp. OIcel þeir ‘they’ (masc.) (early runic þaiʀ, þeiʀ, etc.) < PGmc *þai + z ‘those’; cp. OE þā ‘those’, Go þai, OFris thā, OHG dē.  As well as the form, the use of the pl. demon. as 3 pl. personal pronouns is distinctively Norse (see further Miller 2010: 157 and 2012: XX).

PGmc Ancestor

*þai + z 

Proposed ON Etymon (OIcel representative)

þeir ‘they’
(ONP þeir (CV))

Other Scandinavian Reflexes

early runic þaiʀ, þeiʀ, etc., Far teir, Icel þeir, Norw dei, Dan de, OSw þē(r), Sw de

OE Cognate

þā ‘those’

Phonological and morphological markers

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

Summary category

A1*c

Attestation

Mainly N/EM in early ME (already invariable form in Orrm), but widespread by late ME: on the medieval distribution of the Norse-derived third person pronouns (and their variant spellings), see esp. Samuels 1972: 70–2, Ritt and Morse-Gagne, LAEME map 00002331 and LALME dot map 28.

Occurrences in the Gersum Corpus

Gaw 50, 69, 71, etc.; Pe 80, 94, 509, etc.; Cl 9, 10, 11 etc.; Pat 13, 15, 16 etc.; Erk 9, 43, 45 etc.; WA 1, 3, 13 etc.

McGillivray reads a further instance of þay at Gaw 1445 (<hay þay>, for others' <hay hay>).

Bibliography

MED thei (pron.) , OED they (pron., adj., adv. and n.) , Dance þay, Bj. 50, de Vries þeir, Mag. þeir, AEW ðā (3)