Where does “undicennio” come from?
undicennio (Italian) comes from Italian -ennio, from Latin -ennium, from Latin -ennis, from Latin annus, from Proto-Italic atnos, from Proto-Indo-European h₂et-no-, from Proto-Indo-European h₂et- — grass.
undicennio (Italian): A period of eleven years
Ancestry of “undicennio”, step by step