A four-letter word is on a lot of people’s minds these days. That word is “shot,” as in, “I want my covid-19 shot! How do I get my covid-19 shot?”

Jon Simon was curious about that word. He is a 67-year-old word researcher who lives in Silver Spring, Md., and has done work for the Oxford English Dictionary.

As Jon knows, words have to come from somewhere. For example, “vaccine” is derived from the Latin word for “cow,” a reminder that Edward Jenner used pus from a cowpox infection — a disease that affected cows and the milkmaids who worked with them — to prevent smallpox.

But why do we call an injection a “shot”?

“It’s not necessarily obvious,” said Jon. So, using online newspaper and book databases, he set out to trace the word back to its origins.

What Jon found is that this common word — employed today in a mostly positive context — first gained widespread use to describe the depths of drug addiction. That drug: morphine.

Opium had been smoked for centuries. Morphine had been swallowed in tablet form or drunk in solution. Then in an 1855 paper, Scottish physician Alexander Wood described a method of introducing morphine directly to the part of the body in pain. Wood used a syringe and fine-bore needle, what we know as a hypodermic needle. (Hypo: under. Derma: skin.)

This made treatment easier, but it also invited misuse.

One of the earliest “shot” mentions Jon found was in the San Francisco Chronicle of March 2, 1889. A story headlined “The Hypo-Gun. How Morphine Victims Are Fed” describes the scene inside a drug house. The quotation marks and explanations suggest the writer thought much of this would be new to the reader:

“The morphine victim is cared for there — as long as he has money. In all the houses frequented by the ‘fiends’ is a man or a woman who sells the drug and injects it for a small sum. This useful person is called the ‘gunner,’ the syringe is termed the ‘gun,’ and administers to the fiend an injection, that is ‘a shot,’ for which he is paid 5 cents.”

It looks like “shot” comes from “gun,” the euphemism for the apparatus that delivered the morphine.

“Shot” had a negative connotation in the waning years of the 19th century, used often in conjunction with morphine addiction:

“Defendant claimed and testified . . . that he was at home, about a half-block from Rathja’s saloon, administering what he calls a ‘shot’ of morphine to one Boyden.” (From “Reports of Cases Determined in the Supreme Court of the State of California,” 1892.)

“There is a place in San Francisco where victims of the insidious morphine habit go for ‘treatment’ daily . . . Men saunter in for a ‘shot’ as other men drop in at a saloon on the way to business for a morning bracer.” (From “Quarterly Journal of Inebriety,” 1894.)

I asked Jon what was going through his mind as he found reference after reference to drug dens.

“Some of it is very sad — and horrifying, even — but mostly it is the thrill of the hunt,” he said. “I’ll admit to that.”

Eventually, the negative connotations of “shot” were forgotten — or subsumed by the more beneficial ways a hypodermic needle can be used.

Jon was also curious about the phrase “a shot in the arm.” It came later. One of the earliest references he found was in the March 2, 1918, issue of Investment Weekly: “However unsound economically Mr. McAdoo’s War Finance Corporation may be, it is bound to have certain effects which the investor cannot afford to overlook. The first of these will be a great stimulus of business — the same sort of stimulation perhaps which results from a ‘shot in the arm’ — but very real stimulation nevertheless.”

While “shot” started out with negative connotations, here was “a shot in the arm” used positively.

“Exactly,” said Jon. “It’s very weird how that worked out.”

Inspired by Jon, I went down the rabbit hole of early morphine coverage. What’s clear is that people discovered pretty quickly that morphine was uniquely addictive — and that overprescribing it could lead to problems.

In 1887, a reporter for the San Francisco Chronicle wrote:

“If morphine is resorted to so constantly by the medical profession for the relief and cure of disease, it necessarily follows that the laity must become more or less familiar with its use. A prescription is given containing morphine; it gives infinite relief; the patient does not think it necessary to consult the physician a second time, but on his own responsibility renews the prescription again and again, unaware of the dangerous pit into which he is falling. Sooner or later he awakens to the realization that his disease is cured, but that he is a slave to morphine, without the power to escape.”

If executives at today’s drug companies had looked at old newspapers, a lot of heartache might have been prevented.

Twitter: @johnkelly

For previous columns, visit washingtonpost.com/john-kelly.