placebo (n.)

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" (Psalms cxvi.9, in Vulgate Placebo Domino in regione vivorum), from Latin placebo "I shall please," future indicative of placere "to please" (see please).

Medical sense is recorded by 1785, "a medicine given more to please than to benefit the patient." Placebo effect is attested from 1900.