Etymology

Always derived from ON, cp. OIcel angra 'to distress, grieve, trouble (etc.)'. The ON wk. v. is formed on the n. angr, also borrowed into English (see further anger (n.)), though attested slightly later in ME. The -r of the stem indicates an original neut. es-stem (PGmc *angez- > *angaz-, cognate with Lat angor 'strangling, anguish’). The same root is found in the OE adj. ange ‘anxious, painful, distressing’ (< PGmc *angu-; cp. OIcel ǫngr, Go aggwus, OHG ango) and the OE adj. enge ‘narrow; anguished; oppressive’ (< the variant *angwja- with i-mutation; cp. OFris enge, OS engi, OHG angi, engi, MDu enghe), but there is no sign of an es-stem outside of Scandinavian, and ON is therefore the most likely source of the ME. 

PGmc Ancestor

*angez- 

Proposed ON Etymon (OIcel representative)

angra 'to distress, grieve, trouble (etc.)'
(ONP angra (vb.))

Other Scandinavian Reflexes

Far angra, Icel angra, Norw angra, ODan angre, Dan angre, OSw angra, Sw ångra

OE Cognate

cp. ange (adj.) 'anxious, painful, distressing', enge (adj.)'narrow, anguished, oppressive'

Phonological and morphological markers

Summary category

C1

Attestation

First attested in Orrm, and more common and widespread in ME from the 14c. and 15c. See also anger (n.).

Occurrences in the Gersum Corpus

WA 837

Bibliography

MED angren (v.) , OED anger (v.) , HTOED , Bj. 200, de Vries angra, Mag. angur (2), Heid. angu-, Orel *anʒaz, Kroonen *angaza-