unbreakably

Etymology tree
Proto-Indo-European *né
Proto-Indo-European *n̥-
Proto-Germanic *un-
Proto-West Germanic *un-
Old English un-
Middle English un-
English un-
Proto-Indo-European *bʰreg-der.
Proto-Germanic *brekaną
Proto-West Germanic *brekan
Old English brecan
Middle English breken
English break
Proto-Indo-European *-tḗr
Proto-Indo-European *-dʰlom
Proto-Indo-European *-dʰlis
Proto-Italic *-ðlis
Latin -bilis
Latin -ābilis
Old French -ablebor.
Middle English -able
English -able
English unbreakable
Proto-Indo-European *leyg-der.
Proto-Germanic *līkąder.
Proto-Germanic *-līkaz
Proto-Germanic *-ê
Proto-Germanic *-līkê
Proto-West Germanic *-līkē
Old English -līċe
Middle English -ly
English -ly
English unbreakably
From unbreakable + -ly.