1670s, "muscle which allows a part to be moved circularly," agent noun from Latin rotare "turn round, revolve" (see rotary). Also compare rotor. General mechanical sense of "one who or that which rotates" is by 1772. Related: Rotatory.
1731, from Medieval Latin rotarius "pertaining to wheels," from Latin rota "a wheel, a potter's wheel; wheel for torture," from PIE root *ret- "to run, to turn, to roll" (source also of Sanskrit rathah "car, chariot;" Avestan ratho; Lithuanian ratas "wheel," ritu "I roll;" Old Irish roth, Welsh rhod "carriage wheel"). The root also forms the common West Germanic word for "wheel" (originally "spoked wheel"): Old High German rad, German Rad, Dutch rad, Old Frisian reth, Old Saxon rath.
The international service club (founded by Paul P. Harris in Chicago in 1905) is so called from the practice of clubs entertaining in rotation. Hence Rotarian (1911).