The story
- Proto-Indo-European*né
- Proto-Indo-European*n̥-
- Proto-Germanic*un-
- Proto-West Germanic*un-
- Old Englishun-
- Middle Englishun-
- Englishun-
- Englishbreachable
- English todayunbreachable
From un- + breachable.
Etymology from Wiktionary, CC BY-SA 4.0.