hadn't

The story

1. Englishhad
2. Proto-Indo-European*né
3. Proto-Germanic*ne
4. Proto-West Germanic*ne
5. Proto-Indo-European*h₂ey-
6. Proto-Indo-European*h₂óyu
7. Proto-Germanic*aiwaz
8. Proto-West Germanic*aiw
9. Proto-West Germanic*naiw
10. Proto-Indo-European*wekʷ-
11. Proto-Indo-European*wékʷtisder.?
12. Proto-Indo-European*weǵʰ-
13. Proto-Indo-European*wéǵʰtisder.?
14. Proto-Germanic*wihtą
15. Proto-West Germanic*wiht
16. Proto-West Germanic*naiwwiht
17. Old Englishnāwiht
18. Old English*nōht
19. Middle Englishnought
20. Middle Englishnot
21. Englishnot
22. English todayhadn't

From had + -n't.