Lewis & Short

2. tranquillo, āvi, 1, v. a. [tranquillus], to make calm or still, to calm, still.

  1. I. Lit. (very rare; syn. sereno): mare tranquillatur oleo, Plin. 2, 103, 106, § 234.
    1. B. Transf.: vultum, i. e. to clear up, brighten, Plaut. Capt. 1, 2, 21.
  2. II. Trop., to calm, compose, tranquillize (class.): ut aut perturbentur animi aut tranquillentur, Cic. Top. 26, 98: animos, id. Fin. 1, 16, 50: tranquillatis rebus Romanis, Nep. Att. 4, 5: quid pure tranquillet, honos an dulce lucellum, Hor. Ep. 1, 18, 102.