‘slow’(Modern English dull)
Formally this word could be native: cp. ME dill next to ME dull, supposing a parallel OE formation *dyl, *dylle < *dulja- to the attested OE dol (< PGmc *dula-, cp. OS dol, OFris dol, OHG tol) on the root dul- (so OED and MED; see also Holthausen 1923: 135 who argues that *dylle is also the source of PDE dull), but McGee (363) argues that the dial distribution points to Scandinavian influence. While there is no directly corresponding ON adj., there is a n. formed on the same root represented by OIcel dul (f.), dulr (m.) ‘delusion, concealment, disguise’ (sb. cognate of OE dol etc.). Orel and Kroonen both connect the adj. with the st. v. *dwelan- ‘to err’, hence also the ON wk. v. represented by OIcel dylja wk. v. ‘to keep uninformed’ < *duljan-, i.e. an i-mutated form in dyl- closest to that apparently behind dylle. Cp. also the part. údulðr ‘not unaware, knowing’.
Proposed ON Etymon (OIcel representative)
dul, dulr (n.) ‘delusion, concealment, disguise’; cp. dylja (v.) ‘to keep in ignorance, conceal’, údulðr ‘not unaware, knowing’
(ONP dul (sb.), dulr (sb.))
Other Scandinavian Reflexes
Far duli, dulur, Icel dul, Norw dul, OSw dul
*dyl, *dylle; cp. dol ‘foolish, silly’
Phonological and morphological markers
MED’s citations are all N/EM, see further McGee 363. OED notes that spellings in dul- are unusual before c. 1350.
Occurrences in the Gersum Corpus