Etymology

The most plausible source is VAN *þráʀ, cp. OIcel þrár ‘stubborn, obstinate’ < PGmc *þrawa-; cp. on the same root the nouns OE þrawu, þrēa ‘threat, oppression’, OIcel þrá, OS thrā-werk, OHG drawa, drōa; in that case the form of an OE adj. would have developed differently following loss of historical */w/ (we should expect OE *þrēa(w); cp. e.g. PGmc *xrawa- > OE hrēaw ‘raw’). There is also the possibility of input from the homophone VAN *þráʀ, cp. OIcel þrár ‘decomposed, stale’, probably < PGmc *þranxa- (cp. OE þrōh ‘rancid’, MHG adv. drāhe), in which case loan from ON is guaranteed by early ME /ɑ:/ (later /ɔ:/) < ON /ɑ:/, rather than OE /o:/ < PGmc */an/ before /x/, though probably not by the loss of final -/x/.

PGmc Ancestor

*þrawa-

Proposed ON Etymon (OIcel representative)

þrár ‘stubborn, obstinate’ 
(ONP þrár (2) (adj.))

Other Scandinavian Reflexes

Far tráur, Icel þrár, Norw trå, ODan trå, Sw dial trå(r) 

OE Cognate

þrawu, þrēa (n.) ‘threat, oppression’

Phonological and morphological markers

ON loss of */w/ medially and compensatory lengthening

[ON / ɑ:/ < PGmc */anx/] (may not be applicable)

[ON loss of */x/ finally] (may not be applicable)

Summary category

A1c

Attestation

Predominantly N and E and alliterative (inc. WPal), but occasionally elsewhere. Now obsol., but EDD cites MnE use in Yks. dial. 

Occurrences in the Gersum Corpus

Gaw 645, 1713, 1751 etc.; Pe 344, 868; WA 554, 1246, 1631

Formulaic collocation with ME þryuen is recurrent.

Bibliography

MED thrō (adj.) , OED thro, thra (adj.1 and adv.) , HTOED , EDD thro (adj.), Dance þro (a)(adj.), Bj. 106–7, 181, de Vries þrár, Mag. þrár (1) and (2), Heid. þrawa-; þranha-, Orel *þrawaz; *þranxaz, Kroonen *þrawō-, AEW ðrēa; ðrōh