1740, from French sénatorial or from Latin senatorius "pertaining to a senator" or formed in English from senator + -al (1). Earlier adjectives were senatory (1520s), senatorian (1610). Related: Senatorially.
c. 1200, "member of an (ancient) senate," from Old French senator (Modern French sénateur), from Latin senator "member of the senate," from senex "old; old man" (from PIE root *sen- "old"). An Old English word for one was folcwita. As "member of a (modern) governing body" from late 14c.; specifically in U.S. use from 1788. Fem. form senatress attested from 1731. The Senators was the name of the professional baseball team in Washington, D.C., from 1891 to 1971.