Etymology

An adv. formed on an adj. derived from OE hāl ‘healthy, whole’ (etc.) (PGmc *xaila-, cp. OIcel heill etc.) + the suffix OE -sum (cp. OFris -sum and (with different Ablaut) Go -sams, OIcel -samr, OS, OHG -sam).  No such form *hālsum is attested in OE, but cp. OHG heilsam, OS hēlsamo (adv.) and OIcel heilsamr ‘salutary’ (the adv. heilsamliga is also recorded). ON imput is rarely adduced (TGD and GDS; MED points to the OHG and OIcel forms as cognates only); OED suggests that the word in N ME (to which it gives a separate entry, s.v. halesome adj.) is ‘reinforced from Old Norse’.  

PGmc Ancestor

*xaila-

Proposed ON Etymon (OIcel representative)

heilsamr ‘salutary’; cp. heilsamliga 'uprightly'
(ONP heil-samr (adj.); heil-sam-liga (adv.))

Other Scandinavian Reflexes

Far heilur, Icel heill, Norw heil, Dan hel, Sw hel

OE Cognate

hāl ‘healthy, whole’ (etc.)

Phonological and morphological markers

Summary category

CCC1a

Attestation

The adj. holsum is very widespread in ME;  MED records several attestations of the adv. (from Trin.Hom. onwards), without evident dial bias.

Occurrences in the Gersum Corpus

Gaw 1731

Read halsumly by Madden.

Bibliography

hōlsomlī(che (adv.) , OED wholesomely (adv.) , HTOED , Dance holsumly, de Vries heill (2), Mag. heill (2), Bj-L. heil, Heid. haila-, Orel *xailaz (II); *samaz, Kroonen *haila-, AEW hāl, hǣl; sum (2)