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

The Collaborative International Dictionary of English v.0.48:

Tranquilizing \Tran"quil*i`zing\, Tranquillizing
\Tran"quil*li`zing\, a.
   Making tranquil; calming. " The tranquilizing power of time."
   --Wordsworth. -- Tran"quil*i`zing*ly or
   Tran"quil*li`zing*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tranquillizing
    adj 1: tending to soothe or tranquilize; "valium has a
           tranquilizing effect"; "took a hot drink with sedative
           properties before going to bed" [syn: ataractic,
           ataraxic, sedative, tranquilizing,
           tranquillizing, tranquilising, tranquillising]