nonargumental

The story

1. Proto-Indo-European*né
2. Proto-Germanic*ne
3. Proto-Indo-European*ís?
4. Proto-Indo-European*h₁óynos
5. Proto-Germanic*ainaz
6. Proto-Germanic*nainaz
7. Proto-West Germanic*nain
8. Old Englishnān
9. Middle Englishnon
10. Old Englishnān
11. Old Englishnān-
12. Middle Englishnon-
13. Englishnon-
14. Proto-Indo-European*h₂erǵ-
15. Proto-Indo-European*h₂érǵus
16. Proto-Italic*argusder.?
17. Latinarguō
18. Latinargūmentum
19. Anglo-Normanarguement
20. Middle Englishargument
21. Englishargument
22. Proto-Indo-European*h₂el-
23. Proto-Indo-European*h₂élisder.?
24. Englishargumental
25. English todaynonargumental

From non- + argumental.