Etymology

Formally, garysoun certainly represents a loan from AF garisun (itself ultimately a derivation on PGmc *warjan- ‘to defend’) which is recorded in a range of senses (see AND) with an underlying sense ‘something that will protect/keep/provide for’. AND’s sense 3 (esp. ‘livelihood, income’) is fairly close to MED's sense 1a. (‘treasure, wealth; gift, donation, tribute, payment’), and thus some authorities seek no further input (MED, OED, Morris, GDS and Kullnick 23). But there is a good case for semantic influence from ME gersum, derived from ON, cp. OIcel gersemi (etc.) (or gør-) ‘costly thing, jewel, treasure (literal and figurative)’ (see further gart). Its senses overlap with those recorded for ME garisoun, some of whose meanings seem somewhat closer to those of the ON word than those of the Fr (e.g. ‘keepsake, souvenir’ has more in common with the ON idea of ‘treasured object’ than the Fr ‘livelihood, income’ etc.) . The association between the two words is evident in the recurring collocation of garisoun with golde as well as gersum with golde (cp. the common pairing of ON gersimi with gull (Olszewska 1933: 81)).

PGmc Ancestor

*warjan- 

Proposed ON Etymon (OIcel representative)

cp. gersemi, gersimi, giersom (etc.) (or gør-) ‘costly thing, jewel, treasure (literal and figurative)’ 
(ONP cp. ger-semi (sb.))

Other Scandinavian Reflexes

cp. Far gersemi, gersimi, Icel gersemi,  Dan gørsum, OSw gärsimi, görsum, görsim

OE Cognate

Phonological and morphological markers

Summary category

FCC3

(FCC4)

Attestation

MED cites a handful of occurrences under its sense 1a. (‘treasure, wealth; gift, donation, tribute, payment’), earliest from a1300 Floris (Vit D.3), and including a number in collocation with golde, mainly (but not exclusively) from alliterative verse.

Occurrences in the Gersum Corpus

Gaw 1255, 1807, 1837; WA 1074, 1662, 1809 etc.

Bibliography

MED garisoun (n.) , OED garrison (n.) , Bj. 152, AND garisun, HTOED , Dance garysoun, de Vries gersemi, gørsemi, Mag. gersamlega, FEW (Germanismes) *warjan, DEAF garir (v.)