Many words change their meaning with the years, and in some cases the new meaning is the opposite of the old, or at least at a tangent to it. A well known example is prestigious, which originally meant deceitful and now means esteemed. I am not sure, but I think that placebo may be in that camp.
The standard etymology is that placebo is the first person future indicative of the Latin word placeo, I please—that is, placebo=I shall please. But I don’t think it’s as simple as that.
The word first entered the English language through its erroneous use in a Latin translation of verse 9 of Psalm 116, which in Hebrew transliteration is “et’halekh liphnay adonai b’artzot hakhayim.” This is correctly translated in the King James version as “I will walk before the Lord in the land of the living,” but the Latin version in the Vulgate of St Jerome is “Placebo Domino in regione vivorum.” This (“I shall please the Lord in the land of the living”) is not a correct translation of the Hebrew, but perhaps Jerome reasoned that anyone who walked before (or as the Revised English Bible has it “in the presence of”) the Lord would please Him, and used the word placebo for the sake of euphony or metre (the Latin line happens to be a dactylic pentameter, like the dactylic hexameters used in Greek and Latin epic verse, such as Virgil’s Aeneid).
Because the verse was used in the Vespers of the Office for the Dead, the word placebo, with which it began, became in the thirteenth century the name of that service. And because some people attended the service and sang the Placebo, hoping to be rewarded by a dead person’s relatives, the word came to mean a sycophant. As Chaucer wrote in The Parson’s Tale: “Flatterers are the Devil’s chaplains, always singing Placebo.”
Then suddenly the word appeared in the “enlarged and improved” second edition of George Motherby’s New Medical Dictionary of 1785 (not having appeared in the first edition of 1775), where it was defined as “a common place method or medicine.” Why placebo came to have that meaning at that time is not clear. Common-place (usually spelt with a hyphen in those days) meant then, as it does now, ordinary, unoriginal, trivial, and so Motherby’s definition suggests neither the modern meaning of placebo nor its traditional derivation (“I shall please”). However, the word “placere” also meant “to be popular,” so a placebo may originally have been a drug that was of sufficient efficacy, or perceived efficacy, to be popular.
The word next appears in 1811 in Hooper’s Medical Dictionary: “any medicine adapted more to please than benefit the patient.” This sounds closer to our modern use of the word, and it is possible that a change in meaning may have taken place at about the turn of the nineteenth century, perhaps through a misunderstanding over the original derivation; or perhaps through a form of medical snobbery, the assumption being that anything popular couldn’t be of much value; or possibly harking back to the old idea of sycophancy—that is, a medicine that flattered the user into believing it to be effective.
So perhaps from being a popular medicine with a useful if minor effect, a placebo became a medicine without any effect at all, or not one that you could rely on. More medical examples of the use of the word before 1785 or at around the turn of the nineteenth century might throw some light on the matter.
Footnotes
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