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early 13c., name given to the rite of Vespers of the Office of the Dead, so called from the opening of the first antiphon, "I will please the Lord in the land of the living" (Psalm cxiv:9), from Latin placebo "I shall please," future indicative of placere "to please" (see please). Medical sense is first recorded 1785, "a medicine given more to please than to benefit the patient." Placebo effect attested from 1950.
placebo pla·ce·bo (plə-sē'bō)
n. pl. pla·ce·bos or pla·ce·boes
A substance containing no medication and prescribed or given to reinforce a patient's expectation to get well.
An inactive substance or preparation used as a control in an experiment or test to determine the effectiveness of a medicinal drug.
A substance containing no active drug, administered to a patient participating in a medical experiment as a control.
Note: Those receiving a placebo often get better, a phenomenon known as the placebo effect.