The story
- Proto-Indo-European*né
- Proto-Indo-European*n̥-
- Proto-Germanic*un-
- Proto-West Germanic*un-
- Old Englishun-
- Middle Englishun-
- Englishun-
- Proto-Italic*wre-
- Latinre-
- Old Frenchre-
- Proto-Indo-European*ǵʰreh₁d-
- Proto-Germanic*grētaną
- Frankish*grātan
- Old French*greter
- Old Frenchregreter
- Middle Englishregretten
- Englishregret
- Proto-Indo-European*pleh₁-
- Proto-Indo-European*pl̥h₁nós
- Proto-Germanic*fullaz
- Englishregretful
- Englishregretfully
- English todayunregretfully
From un- + regretfully.
Etymology from Wiktionary, CC BY-SA 4.0.