Tranquilize, Tranquillize, v. t. [imp. & p. p. Tranquilized or Tranquilliized; p. pr. & vb. n. Tranquilizing or Tranquillizing.]
Etymology: [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.
Syn. -- To quiet; compose; still; soothe; appease; calm; pacify.
Shades of meaning
Fernald has no entry of its own for tranquilize; he lists it under allay.
Where he writes about it:
We allay suffering by using means to soothe and tranquilize the sufferer; we alleviate suffering by doing something toward removal of the cause, so that there is less to suffer; where the trouble is wholly or chiefly in the excitement, to allay the excitement is virtually to remove the trouble; as, to allay rage or panic; we alleviate poverty, but do not allay it. — from allay
Pacify, directly from the Latin, and appease, from the Latin through the French, signify to bring to peace; to mollify is to soften; to calm, quiet, or tranquilize is to make still; compose, to place together, unite, adjust to a calm and settled condition; to soothe (originally to assent to, humor) is to bring to pleased quietude. — from allay