antiactivator

The story

1. Proto-Indo-European*h₂ent-
2. Proto-Indo-European*h₂énts
3. Proto-Indo-European*h₂énti
4. Ancient Greekᾰ̓ντῐ́ (ăntĭ́)
5. Ancient Greekἀντι- (anti-)
6. Englishanti-
7. Proto-Indo-European*h₂eǵ-
8. Proto-Indo-European*h₂éǵeti
9. Proto-Italic*agō
10. Latinagō
11. Ancient Greekἐνεργητικός (energētikós)
12. Latināctīvu
13. Old Frenchactif
14. Middle Englishactyf
15. Englishactive
16. Englishactivate
17. Englishactivator
18. English todayantiactivator

From anti- + activator.