cyclobutene

The story

1. Proto-Indo-European*kʷelh₁-redup.
2. Proto-Indo-European*kʷékʷlos
3. Proto-Hellenic*kʷókʷlos
4. Proto-Hellenic*kúklos
5. Ancient Greekκύκλος (kúklos)
6. Late Latincyclus
7. Middle Englishcicle
8. Englishcycle
9. Ancient Greekκύκλος (kúklos)
10. Ancient Greekκυκλῐκός (kuklĭkós)
11. Classical Latincyclicu
12. Middle Frenchcyclique
13. Frenchcyclique
14. Classical Latincyclicus
15. Englishcyclic
16. Englishcyclo-
17. Proto-Indo-European*gʷṓws
18. Proto-Hellenic*gʷous
19. Ancient Greekβοῦς (boûs)
20. Proto-Indo-European*tewh₂-
21. Proto-Indo-European*tuh₂rós
22. Proto-Hellenic*tūrós
23. Ancient Greekτῡρός (tūrós)
24. Ancient Greekβούτῡρος (boútūros)
25. Englishbut-
26. Englishbutene
27. English todaycyclobutene

From cyclo- + butene.