Back in the 13th century, the term placebo didn’t call to mind clinical trials, sugar pills, or anything remotely medical. Instead, if you were a member of the Roman Catholic Church, it would’ve likely made you think of God and death.
As Merriam-Webster explains, placebo means “I will please” in Latin, and it happened to be the first word of the responsorial psalm that began "Vespers" in the "Office for the Dead"—an evening prayer that Catholics recited for people who had died. (The full line translates to “I will please the Lord in the land of the living.”) Before long, people started using placebo to refer to the entire prayer.
By the following century, however, imaginative English speakers had given placebo a secondary definition that echoed its literal Latin meaning: If you said someone was singing, making, or playing placebo, you were implying that they were flattering someone in a sycophantic or servile way. You could even just cut to the chase and call the person themselves a placebo, which, according to the Oxford English Dictionary, meant “a flatterer, a sycophant, [or] a parasite.” We have Geoffrey Chaucer to thank for the earliest known instance of this; in “The Merchant’s Tale” (from The Canterbury Tales), he named one of the characters “Placebo.” Placebo, unsurprisingly, spends a lot of time telling his older brother exactly what he wants to hear, while a third brother, Justinus, gives much better advice.
It makes sense that the word placebo—flattery with the intent to make someone feel good, even if it’s not necessarily true—eventually landed in medicine, where it came to define any drug or treatment meant to make someone feel good, even if it technically had no medical potency. But it didn’t get there until well into the 18th century.
“Where a placebo merely is wanted, the purpose may be answered by means, which, although perhaps reduced under the materia medica, do not, however, deserve the name of medicines,” physician Andrew Duncan wrote in his 1770 book Elements of Therapeutic.
The full phrase placebo effect didn’t become common until the early 1900s. The 20th century also saw the birth of placebo’s evil twin—nocebo (Latin for “I will harm”), which describes a medically worthless or empty treatment that somehow causes a patient to feel worse. Needless to say, nocebo hasn’t exactly caught on in the same way that placebo has.