c.1200, "legal and administrative body of ancient Rome," from Old French senat or Latin senatus "highest council of the state in ancient Rome," literally "council of elders," from senex (genitive senis) "old man, old" (see senile). Attested from late 14c. in reference to governing bodies of free cities in Europe; of national governing bodies from 1550s; specific sense of upper house of U.S. legislature is recorded from 1775.