Etymology

Always identified with the MnE dial word rissom. The closest comparanda are Scandinavian derivational forms on the PGmc root *xrīs- 'brushwood' with a nasal suffix, which is unparalleled elsewhere (cp. OIcel hrís, OE hrīs 'twig, branch', OFris hrīs 'switch, rod', MLG rīs 'twig, branch', OHG hrīs, rīs), cp. Sw dial resma, resme 'ear of oats' and, with different vocalism, Dan rusme, rosme  'head of oats' (see further Torp-Falk and note the less common MnE variant ruzzom cited by OED3 and EDD from Yks.). 

PGmc Ancestor

*xrīs-

Proposed ON Etymon (OIcel representative)

(ONP )

Other Scandinavian Reflexes

Sw dial resma, resme; cp. Dan rusme, rosme

OE Cognate

Phonological and morphological markers

Summary category

C1c

Attestation

This is the only ME instance of the word, which is otherwise cited by OED3 and EDD from N dial of MnE.

Occurrences in the Gersum Corpus

WA 3060

Skeat WA reads <risonis>, but notes that <risoms> is also possible, and this is the reading given by TPD.

Bibliography

MED rison (n.) , OED3 rissom (n.) , HTOED , EDD rissom (sb.), Orel *xrīsan, Kroonen *hrīsa-, Torp-Falk Rusme, Hellquist resma