means A treatment with no active medicinal ingredient, given to please or reassure a patient — and which can produce real effects simply because the patient expects it to.

from Straight from Latin: placebo means 'I shall please,' the future tense of placere, 'to please.' The medical sense was preceded by a darker one — in the medieval Church, 'placebo' was the opening word of the Vespers for the Dead (from a Psalm line, 'I shall please the Lord'), and 'to sing placebo' came to mean a flatterer, a sycophant who tells you what you want to hear. That whiff of pleasing-without-substance carried straight into medicine, where by the late 18th century a placebo was a remedy given more to satisfy the patient than to cure them.