cochairman

The story

1. Proto-Indo-European*ḱe?
2. Proto-Indo-European*ḱóm
3. Proto-Italic*kom
4. Proto-Italic*kom-
5. Latincon-
6. Latinco-
7. Englishco-
8. Proto-Indo-European*ḱóm
9. Proto-Indo-European*ḱm̥-th₂der.?
10. Proto-Hellenic*kətá
11. Ancient Greekκατά (katá)
12. Proto-Indo-European*sed-
13. Proto-Indo-European*sedreh₂
14. Proto-Hellenic*hédrā
15. Ancient Greekἕδρα (hédra)
16. Ancient Greekκᾰθέδρᾱ (kăthédrā)
17. Latincathedra
18. Old Frenchchaiere
19. Middle Englishchayere
20. Englishchair
21. Proto-Indo-European*mon-
22. Proto-Germanic*mann-
23. Proto-West Germanic*mann
24. Old Englishmann
25. Middle Englishman
26. Englishchairman
27. English todaycochairman

From co- + chairman.