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"tending to calm or soothe," early 15c., from Medieval Latin sedativus "calming, allaying," from sedat-, past participle stem of sedare, causative of sedere "to sit" (see sedentary). The noun derivative meaning "a sedative drug" is attested from 1785. Hence, "whatever soothes or allays."
sedative sed·a·tive (sěd'ə-tĭv)
Having a soothing, calming, or tranquilizing effect; reducing or relieving anxiety, stress, irritability, or excitement. n.
An agent or a drug that produces a soothing, calming, or tranquilizing effect.