The story
- 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*lewh₁-
- Proto-Indo-European*lewHs-
- Proto-Germanic*leusaną
- Proto-Germanic*lausaz
- Englishregretless
- English todayregretlessness
From regretless + -ness.
Etymology from Wiktionary, CC BY-SA 4.0.