Walpole's own definition of serendipity excludes what you were looking for
His 1754 letter to Horace Mann built the word from three fairy-tale princes who read a lost camel's whole history in footprints and grazed grass, clues nobody asked them to notice.
The faculty of finding valuable or agreeable things not sought for.1
The Letter That Named an Accident
On 28 January 1754, Horace Walpole wrote to his friend Horace Mann in Florence about a small triumph. Dipping into an old book of Venetian coats of arms, he had turned up the exact detail he needed to explain a portrait Mann had just sent him. He had no name for how he kept finding things this way, so he made one.2
“This discovery, indeed, is almost of that kind which I call Serendipity, a very expressive word,” he told Mann. He explained where it came from: a “silly fairy tale, called the three Princes of Serendip.”2 Its heroes, he wrote, “were always making discoveries, by accidents and sagacity, of things which they were not in quest of.”2 Serendip was the old name for Ceylon, today's Sri Lanka.1
Walpole's own example, retold from memory, had the princes reason out a lost animal's condition from the roadside. Grass cropped short on one side only meant it was blind in that eye. Scattered footprints meant it dragged a foot. He remembered the animal as a mule. The fairy tale's animal was a camel.2
One line in the letter does more work than the anecdote. “No discovery of a thing you are looking for comes under this description,” Walpole wrote.2 Sagacity earns the name in his own account only when it turns up something nobody sent it after.
Two Centuries Before the Word Caught On
Walpole's word stayed close to his own circle for a long time. Merriam-Webster's first recorded use of the noun is 1754 itself, drawn straight from this letter, and no earlier or wider use is on record.1 Robert Merton and Elinor Barber, whose study traces the word's own social history, describe what came next as “the boom serendipity was to have in the middle of the twentieth century.”2 That was the point where scientists and writers outside his own circle picked it up for good.
What Fleming Almost Threw Away
The clearest modern instance of what Walpole meant sits in a London laboratory on 3 September 1928. Alexander Fleming returned from holiday to a culture plate ruined by stray mold, the kind of accident most bacteriologists would have rinsed down the sink without a second look.3 Fleming instead noticed that the mold had cleared a ring in the bacteria growing around it. Because he already worked on antibacterial substances, he recognized what the ring meant instead of discarding the plate, and named the substance penicillin.3 The mold was the accident. Reading the ring was the sagacity Walpole meant, the alertness the princes brought to the roadside.
‘Lucky’ Doesn't Cover the Sagacity Half
Merriam-Webster's own record of how people use the word shows both halves still competing. In May 2026, the Forbes columnist Caroline Castrillon wrote about remote work that “serendipity now has to be scheduled.”4 The usage treats a chance meeting as something built, not stumbled into. A Chicago Tribune horoscope column from December 2020 put the other half on record: “The wheel of fortune is blessing you with a day of pleasure and serendipity,” pure luck, no sagacity implied.4
“Lucky” covers only the accident. It has nothing to say about the plate Fleming might have rinsed clean, or the grazing pattern the princes might have walked past like anyone else on that road. Serendipity is the rarer word because it insists on both at once, the accident and the readiness to see it. That is the distinction Walpole reached for before he had even finished explaining his new coinage to Mann.