nonadeca-

The story

1. Proto-Indo-European*h₁néwn̥
2. Proto-Indo-European*h₁newn̥(n)ós
3. Proto-Italic*nowenos
4. Latinnōnus
5. Englishnona-
6. Proto-Indo-European*déḱm̥
7. Proto-Hellenic*dékə
8. Ancient Greekδέκᾰ (dékă)
9. Ancient Greekδεκάς (dekás)
10. Late Latindecas
11. Frenchdéca-
12. Englishdeca-
13. English todaynonadeca-

From nona- + deca-.