The in- infix does not seem to have an etymological ground, but may be a reflex of the English -ing ending of the present participle of severer. See also severauntment.
adv.
1separately, apart from each other:
(
s.xiiim; MS: s.xiiim
)
ele est virgne, ço saciez veritablement, E cheune noit dormum severinement
4683
severinement (s.xiiim)