underactuate

The story

1. Proto-Indo-European*h₁én
2. Proto-Indo-European*h₁entér
3. Proto-Indo-European*h₂én
4. Proto-Indo-European*dʰeh₁-?
5. Proto-Indo-European*(H)n̥dʰí
6. Proto-Indo-European*(H)n̥dʰér
7. Proto-Germanic*under
8. Proto-West Germanic*undar
9. Old Englishunder-
10. Middle Englishunder-
11. Englishunder-
12. Proto-Indo-European*h₂eǵ-
13. Proto-Indo-European*h₂éǵeti
14. Proto-Italic*agō
15. Latinagō
16. Latināctus
17. Latināctuō
18. Medieval Latināctuātus
19. Englishactuate
20. English todayunderactuate

From under- + actuate.