Etymology

The most frequently suggested etymology for this word connects it with OIcel grindill  'destroyer' (i.e. wind, storm) (thus Mätzner, GDS, MED, and TGD (followed by McGee) derives it directly), a poetic weather heiti (and hapax legomenon) probably referring to a storm (LP) and plausibly derived (De Vries, Mag.) from the e-grade of PGmc *grend- ‘grind’, as in OE grindan ‘to grind’ and a range of probably related forms, and hence an underlying sense like ‘that which grinds up, destroys’. TGD explains the fact that the ME word is an adj. rather than a n. by taking gryndel as a back-formation on gryndellayk (ON grindill ‘storm’ + -layk), but ME -layk is usually suffixed to adjectives, not nouns (see further Dance 2003: 429–32). The alternative is to explain gryndel as a previously unattested native adj. derivation on the same *grend- root (as perhaps implied by Mätzner, MED). Formally closer analogues are nominal derivations on ME grind-, like ME gryndel-ston ‘grind-stone’  and it is not difficult to imagine a native adj. formation on the same root acquiring the meaning ‘destructive’ > ‘fierce’ (see further Dance). (2) OED alternatively compares OIcel grimd ‘vehemence, malice, ferocity, fierceness, violence’ (next to the by-form grim(m)ð), a nominal derivation (< PGmc *gremmiþō, cp. OHG grimmida) on the adj. PGmc *gremma- represented by OIcel grimmr ‘grim, stern, horrible, dire, sore; stern, savage’ (etc.), OE grim ‘grim, fierce, cruel, terrible’, OS, OFris grimm, MDu, OHG grim. Derivation from an ON grimd is very attractive semantically, and one can plausibly posit dissimilation of the ON cluster /md/ > /nd/ during adoption into English. However we would again have to assume conversion of a n. into an adj. (as well as the addition of the l-suffix)

PGmc Ancestor

(1) *grend-; (2) *gremmiþō

Proposed ON Etymon (OIcel representative)

(1) grindill 'destroyer'; (2) grim(m)ð ‘vehemence, malice, ferocity, fierceness, violence’
(ONP (2) grimmð (sb.))

Other Scandinavian Reflexes

Far grimd, Icel grimmd

OE Cognate

(1) cp. grindan (v.) ‘to grind’ (2) cp. grim ‘grim, fierce, cruel, terrible’

Phonological and morphological markers

Summary category

DD1c

Attestation

Only in the Gaw MS (once in Gaw, once in Pat). 

Occurrences in the Gersum Corpus

Gaw 2338; Pat 524

Bibliography

MED grindel (adj.) , OED grindel (adj.) , HTOED , Dance gryndel; (1) de Vries grindill, Mag. grindill (1), Bj-L. grind, Seebold grend-a-, Orel *ʒrenðanan, Kroonen *grindan-, Lloyd and Lühr grint (1); grintil, AEW grindan; grinde; grindel, DOE grind; (ge)grind; grindan; grindel, MED grīnden (v.1) , grīndel-stōn (n.) , OED grind (v.1) ; grindle stone (n.) ; (2) de Vries grimd, Mag. grimmur, Bj-L. grim, Heid. gremma-, Orel *ʒremmaz; *ʒremmiþō, LLoyd and Lühr grim, grimmi; grimmida, AEW grim, DOE grim, OED grim (adj. and adv.)