regrettableness

Etymology tree
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
Proto-Germanic *-in-
Proto-Indo-European *-h₂
Proto-Indo-European *-éh₂
Proto-Indo-European *-yéti
Proto-Indo-European *-eh₂yéti
Proto-Indo-European *-h₂ti
Proto-Germanic *-ōną
Proto-Germanic *-inōną
Proto-Indo-European *-dyé-
Proto-Germanic *-atjaną
Proto-Indo-European *-tus
Proto-Germanic *-þuz
Proto-Germanic *-assuz
Proto-Germanic *-inassuz
Proto-West Germanic *-nassī
Old English -nes
Middle English -nesse
English -ness
English regrettableness
From regrettable + -ness.