unregrettable

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-Italic *wre-
Latin re-der.
Old French re-
Proto-Indo-European *ǵʰreh₁d-der.
Proto-Germanic *grētaną
Frankish *grātander.
Old French *greter
Old French regreterbor.
Middle English regretten
English regret
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 regrettable
English unregrettable
From un- + regrettable.