nonserious

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*seh₁-
15. Proto-Italic*sēros
16. Latinsērus
17. Proto-Indo-European*swer-
18. Proto-Italic*swerjos?
19. Latinsērius
20. Proto-Indo-European*h₃ed-
21. Proto-Indo-European*h₃édosder.?
22. Medieval Latinsēriōsus
23. Old Frenchserieux
24. Middle Englishseryows
25. Englishserious
26. English todaynonserious

From non- + serious.