undiagnostic

The story

1. Proto-Indo-European*né
2. Proto-Indo-European*n̥-
3. Proto-Germanic*un-
4. Proto-West Germanic*un-
5. Old Englishun-
6. Middle Englishun-
7. Englishun-
8. Proto-Indo-European*dwóh₁
9. Proto-Indo-European*trísinflu.
10. Proto-Indo-European*dwís
11. Ancient Greekδιά (diá)
12. Ancient Greekδια- (dia-)
13. Proto-Indo-European*ǵneh₃-
14. Proto-Indo-European*ǵn̥h₃sḱéti
15. Proto-Hellenic*gignṓskō
16. Ancient Greekγῐγνώσκω (gĭgnṓskō)
17. Ancient Greekδῐαγῐγνώσκω (dĭagĭgnṓskō)
18. Ancient Greekδῐᾰ́γνωσῐς (dĭắgnōsĭs)
19. Ancient Greekδῐᾰ- (dĭă-)
20. Proto-Indo-European*ǵneh₃-
21. Proto-Indo-European*ǵneh₃tis
22. Ancient Greekγνῶσις (gnôsis)
23. Ancient Greekγνωστῐκός (gnōstĭkós)
24. Ancient Greekδῐαγνωστῐκός (dĭagnōstĭkós)
25. Englishdiagnostic
26. English todayundiagnostic

From un- + diagnostic.