The story
- Proto-Indo-European*teng-
- Proto-Germanic*þankijaną
- Proto-Germanic*þanhtaz
- Proto-West Germanic*þą̄ht
- Old Englishþōht
- Middle Englishthought
- Englishthought
- Proto-Indo-European*lewh₁-
- Proto-Indo-European*lewHs-
- Proto-Germanic*leusaną
- Proto-Germanic*lausaz
- Englishthoughtless
- English todaythoughtlessness
From thoughtless + -ness.
Etymology from Wiktionary, CC BY-SA 4.0.