The story
- Proto-Indo-European*né
- Proto-Indo-European*n̥-
- Proto-Italic*n̥-
- Latinin-
- Middle Englishin-
- Englishin-
- Englishexpress
- Englishexpressible
- English todayinexpressible
From in- + expressible. Piecewise doublet of unexpressible.
Etymology from Wiktionary, CC BY-SA 4.0.