[vcd]
It appears that the Middle English word is a mistranslation of gris e vair ('grey and white fur cut from the red squirrel's winter coat'), interpreting the second element – e vair – as evere, a variant spelling of ovre1 ('work'). Interestingly, this mistranslation is also found in Latin, in the phrase grisus opus (see opus sense 15, 2041c). This re-interpretation is also attested in Anglo-Norman with grisoevere.