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pla·ce·bo (plə-sēbō)
n. pl. pla·ce·bos or pla·ce·boes
a. A substance that has positive effects as a result of a patient's perception that it is beneficial rather than as a result of a causative ingredient.
b. An inactive substance or preparation used as a control in an experiment or test to determine the effectiveness of a medicinal drug.
2. Something of no intrinsic remedial value that is used to appease or reassure another.
3. (plä-chābō) Roman Catholic Church The service or office of vespers for the dead.

[Latin placēbō, I shall please, first person sing. future tense of Latin placēre, to please; see plāk-1 in the Appendix of Indo-European roots. Sense 3, from Late Latin placēbō, I shall please, the first word of the first antiphon of the vespers service (taken from a phrase in the following psalm, placēbō Dominō in regiōne vīvōrum, "I shall please the Lord in the land of the living").]

Word History: Like the word dirge, placebo has its origin in the Office of the Dead, the cycle of prayers traditionally sung or recited for the repose of the souls of the dead. The traditional liturgical language of the Roman Catholic Church is Latin, and in Latin, the first word of the first antiphon of the vespers service is placēbō, "I shall please." This word is taken from a phrase in the psalm text that is recited after the antiphon, placēbō Dominō in regiōne vīvōrum, "I shall please the Lord in the land of the living." The vespers service of the Office of the Dead came to be called placebo in Middle English, and the expression sing placebo came to mean "to flatter, be obsequious." Chaucer, for example, uses the phrase on two occasions. In the Summoner's Tale, a friar offers the following piece of advice: Beth war, therfor, with lordes how ye pleye. / Singeth placebo and 'I shal if I kan,' "Be wary, therefore, how you deal with lords. / Sing 'Placebo' and 'I shall if I can.'" Placebo eventually came to mean "flatterer" and "sycophant." In the 1700s, placebo began to be used of prescriptions written by a physician solely to please a patient, as by satisfying the patient's desire to take medicine. In many cases, the patient would actually benefit, thanks to what became known as the placebo effect. Later, placebo came to refer to neutral substances used in controlled studies testing the effectiveness of medications.

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2022 by HarperCollins Publishers. All rights reserved.

Indo-European & Semitic Roots Appendices

    Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:

    Indo-European Roots

    Semitic Roots

    The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.