Where does “decenni” come from?
decenni (Italian) comes from Italian decennio, from Latin decennium, from Latin decennis, from Latin annus, from Proto-Italic atnos, from Proto-Indo-European h₂et-no-, from Proto-Indo-European h₂et- — grass.
decenni (Italian): plural of decennio
Ancestry of “decenni”, step by step