The Collaborative International Dictionary of English v.0.48:

Tranquilize \Tran"quil*ize\, Tranquillize \Tran"quil*lize\, v.
   t. [imp. & p. p. Tranquilizedor Tranquilliized; p. pr. &
   vb. n. Tranquilizingor Tranquillizing.] [Cf. F.
   tranquilliser.]
   To render tranquil; to allay when agitated; to compose; to
   make calm and peaceful; as, to tranquilize a state disturbed
   by factions or civil commotions; to tranquilize the mind.
   [1913 Webster]

   Syn: To quiet; compose; still; soothe; appease; calm; pacify.
        [1913 Webster] Tranquilizer