Etymology

cp. OIcel þeira (early runic þaiRa, þeRa, etc.), < PGmc *þaizō- ‘of those’; cp. OE þāra, OFris thēra (next to Go þizē, OS thero, OHG dero < the alternative PGmc *þiz-); see further þay.  WA þare may show formal input from the native cognate.

PGmc Ancestor

*þaizō-

Proposed ON Etymon (OIcel representative)

þeira 'their'
(ONP þeir (CV))

Other Scandinavian Reflexes

early runic þaiʀa, þeʀa, Far teirra, Norw deira, OSw þer(r)a, þaira

OE Cognate

þāra ‘of those’

Phonological and morphological markers

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

Summary category

A1*

Attestation

Still predominantly N/EM in late ME, but with a number of occurrences also in East Anglia, the SE and SW: 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 000064117 and LALME dot map 51. On the use of þayr and þayres in Gaw see also Dance 2013: 55–6.  

Occurrences in the Gersum Corpus

Gaw 1359, 1362; WA 2, 3, 5 etc.

Skeat often prints þar(e) or þer for TPD þair(e).

Bibliography

MED their(e (pron.) , OED their (adj. and pron.) , Dance þayr, Bj. 50, de Vries þeir (þeira), Mag. þeirra, AEW ðāra