contrapositive

The story

1. Proto-Indo-European*ḱe?
2. Proto-Indo-European*ḱóm
3. Proto-Italic*kom
4. Proto-Italic*kom-
5. Proto-Italic*komteros
6. Proto-Italic*komterād
7. Latincontrā
8. Englishcontra-
9. Proto-Indo-European*h₂ep
10. Proto-Indo-European*h₂epó
11. Proto-Indo-European*h₂pó
12. Proto-Italic*po
13. Proto-Indo-European*teḱ-
14. Proto-Indo-European*tḱéytibf.
15. Proto-Indo-European*tḱey-
16. Proto-Indo-European*tḱinéti
17. Proto-Italic*sinō
18. Proto-Italic*pozinō
19. Latinpōnō
20. Latinīvus
21. Latinpositivus
22. Old Frenchpositif
23. Englishpositive
24. English todaycontrapositive

From contra- + positive.