This word appears to be an aphetic form of the in Anglo-Norman otherwise unattested noun *arusure (cf. ruser1 and aruser1).
s.
1(of sparks) sprinkling, spraying:
(
MS: s.xiv
)
Aurifaber caminum habeat in summo perforatum, ut fumus possit per cunctos meatus evaporare. Manus altera levi pulsu folles regat, ut intus existens spiritus per fistulas exciens prunas accendat aque crebra asspercio gloss: huevere rosure igni prebeat alimentum
i 189.170 and ii 86.170
[rusure] (s.xiv)