Etymology

Always derived from ON, cp. OIcel illr ‘ill, evil, bad’.  The etymology of this word is thoroughly opaque and much debated and it has been associated, via syncope and assimilation of various sorts, with a number of Gmc roots.  A currently favoured possibility is a PGmc basis *elhila- (> PrN *ilhila-), cp. OHG ilgī ‘hunger, fasting’ (< *elgīn- with stress on the suffix and Verner's Law) (see Bj-L, Falk-Torp s.v. ilde, Nielsen s.v. ild (II) and Kroonen). For less well regarded suggestions see esp. Hellquist s.v. illa, de Vries and Mag.

PGmc Ancestor

?*elhila- 

Proposed ON Etymon (OIcel representative)

illr ‘ill, evil, bad’
(ONP illr (adj.))

Other Scandinavian Reflexes

Far illur, Icel illur, íllur, Norw ill, ODan ild, OSw ilder, Sw ill

OE Cognate

Phonological and morphological markers

[syncope] (possibly diagnostic) (may not be applicable)

[ON consonant cluster assimilation] (possibly diagnostic) (may not be applicable)

Summary category

B1c

Attestation

Generally N and E in ME, but occasionally further afield (incl. PP and Owl & N.), and as a northernism in CT.Rv.

Occurrences in the Gersum Corpus

Cl 272, 864; Pat 8, 203; WA *3593

The WA occurrence is an emendation of MS wondirfull, introduced by TPD (see their 3721n).

Bibliography

MED il(le (adj.) , OED ill (adj. and n.) , HTOED , Dance ille (a)(n.), Bj. 171 (284n.1), de Vries illr, Mag. illur, Bj-L. ill, Kroonen *elhja-