Writing on January 28, 1754, to the British diplomat Sir Horace Mann, Horace Walpole—an antiquarian and son of Prime Minister Robert Walpole—boasted about a recent discovery he had made in an old book of Venetian arms:

This discovery I made by a talisman, . . . by which I find every thing I want, a pointe nommée [at the very moment], whenever I dip for it. This discovery, indeed, is almost of that kind which I call Serendipity.

As Walpole himself was the author of the term, he felt obliged to give Mann its derivation:

I once read a silly fairy tale, called The Three Princes of Serendip [the ancient name for Ceylon, or Sri Lanka]: as their Highnesses travelled, they were always making discoveries, by accidents and sagacity, of things which they were not in quest of: for instance, one of them discovered that a mule blind of the right eye had travelled the same road lately, because the grass was eaten only on the left side, where it was worse than on the right—now do you understand Serendipity?

The word did not appear in the published literature until the early 19th century and did not become well enough known to use without explanation until sometime in the first third of the 20th century. Antiquarians, following Walpole, found use for it, as they were always rummaging about for curiosities, and unexpected but pleasant surprises were not unknown to them. Some people just seemed to have a knack for that sort of thing, and serendipity was used to express that special capacity.

[as applied to Science in American Scientist]