Where does “senatus” come from?

senatus (Latin) comes from Latin -ātus, from Proto-Italic -ātos, from Proto-Indo-European -éh₂tos.

Ancestry of “senatus”, step by step

Step	Language	Word	Meaning
1	Latin	-ātus	—
2	Proto-Italic	-ātos	—
3	Proto-Indo-European	-éh₂tos	—