nonproper

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*per-der.?
15. Proto-Indo-European*pér
16. Proto-Indo-European*pró
17. Proto-Indo-European*pro-
18. Proto-Italic*pro-
19. Proto-Indo-European*preyH-
20. Proto-Indo-European*priHós
21. Proto-Italic*prijos
22. Proto-Italic*proprijos
23. Latinpropriu
24. Anglo-Normanpropre
25. Middle Englishpropre
26. Englishproper
27. English todaynonproper

From non- + proper.