tranquillize, v. t. [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.