Lewis & Short

ăgāso, ōnis, m. [ago, as Sanscr. agas from ag; v. ago],

  1. I. a driver, but esp. one who drives and takes care of horses, a hostler, groom, Plaut. Merc. 5, 2, 11: duo equi cum agasonibus, Liv. 43, 5: agasonem cum equo, Plin. 35, 11, 40, n. 29.
  2. II. Contemptuously, a low servant, lackey: si patinam frangat agaso, Hor. S. 2, 8, 72; Pers. 5, 76.