The story
- Proto-Indo-European*né
- Proto-Indo-European*n̥-
- Proto-Germanic*un-
- Proto-West Germanic*un-
- Old Englishun-
- Middle Englishun-
- Englishun-
- Ancient Greekδῐαγῐγνώσκω (dĭagĭgnṓskō)
- Ancient Greekδῐᾰ́γνωσῐς (dĭắgnōsĭs)
- Latindiagnōsis
- Englishdiagnosisbf.
- Englishdiagnos(e)
- Englishdiagnosable
- Englishundiagnosable
- English todayundiagnosably
From undiagnosable + -ly.
Etymology from Wiktionary, CC BY-SA 4.0.