Ada

It happens more than you'd think. Burgle from burglar, edit from editor, sculpt from sculptor. English is quite happy to work backwards.

Two completely unrelated words that happen to collide in spelling, and your daughter has picked a genuinely good example.

Kidnap is late 1600s, and it's built from kid meaning child, which was already slang, plus nap meaning to snatch or seize. That nap is a variant of nab, the same one still alive in getting nabbed. The word appears in the context of children being seized and shipped to the American colonies as labour, and interestingly the agent noun kidnapper shows up first, with the verb kidnap looking like a back-formation from it.

Napkin is a couple of centuries older and comes into English from Old French nape, a tablecloth, ultimately Latin mappa. English then added the diminutive ending -kin, the one in lambkin and manikin. So napkin is literally a little tablecloth.

The nap on fabric, the fuzzy surface of velvet, is yet a third word from Middle Dutch. Three unrelated naps, one spelling.

Expected, and often large. A first citation is the earliest surviving written use anyone has found, which is a very different claim from the earliest use.

The gap is worst exactly where you noticed it, in slang. Spoken slang goes unwritten by definition until someone has a reason to record it, and the people who wrote things down historically were often not the people using it. When it does first surface it's frequently in a glossary, a court transcript or a novelist quoting speech, all of which mean it was already established.

A rough feel from watching entries get revised: a few years for technical vocabulary that arrives in print immediately, a decade or two for ordinary words, and sometimes fifty years or more for spoken slang.

One useful tell is the tone of the first citation. If the earliest example explains the word to the reader, you're near the beginning. If it uses it casually with no gloss, the word was already common and the real origin is earlier.

It's the OED's earliest citation for the phrase, and it's repeated in most of the serious treatments. If you have a library card, a lot of public library systems give you OED access from home and you can read the citation list yourself, which is more satisfying than taking my word for it.

The dates kill it, which is the cleanest way to settle these.

The earliest use anyone has found of the phrase in print is 1685, in a Scottish sermon by James Durham, complaining about people who go by no rule but the rule of thumb. It's used there exactly the way we use it now, meaning a rough practical guide rather than a precise measure.

The beating story is usually pinned on Francis Buller, an English judge who in 1782 was widely mocked in caricature for supposedly saying a husband could beat his wife with a stick no thicker than his thumb. There's no surviving record of him ruling any such thing, and even if he had, the phrase was already about a century old.

So you have a phrase attested in 1685 and a story dated 1782 that the phrase is supposed to have come from. That's not how time works.

Fix the one-word notes at the point of capture, not at the point of processing. Give yourself six extra words: not just Sarah, but ask Sarah about the March invoice. It costs three seconds in the car and saves you the entire archaeology problem later.

That's a good heuristic generally. Real etymology is full of probably, compare with, and origin obscure. A story with a workshop, a motive and a punchline is usually a story.

Invented, or at least entirely unevidenced. The word comes from Latin sincerus, meaning clean, sound, pure, unadulterated, and it's used of things like unmixed wine and unblemished bodies long before anyone tells the sculpture story.

The honest answer about what's inside sincerus is that nobody is certain. The most commonly repeated proposal breaks it into an element meaning one or single plus a root to do with growth, so roughly of one growth, unmixed. Serious dictionaries flag that as uncertain rather than settled.

What we can say confidently is that no surviving Latin text uses sine cera as a phrase for honest workmanship, and the sound change from sine cera to sincerus isn't the shape Latin words normally take. It appears in English-language moralising sermons and essays much later, which is a very common life cycle for a tidy etymology.

The general method matters more than this particular word, so here's how I'd rank sources when they disagree.

First, prefer sources that quote dated texts over sources that assert. A claim with a newspaper name and a date can be checked. A claim that says probably from an African language cannot.

Second, look for the earliest attestation and ask whether the proposed origin can explain it. If your explanation requires the word to exist in 1790 and nobody can produce it in print before 1839, the explanation has a hole in it.

Third, check whether the story explains the form as well as the meaning. Lots of proposals give you a plausible sound and no account of why it was written as two capital letters.

On OK specifically, the 1839 Boston newspaper trail is the one with actual dated citations behind it. It came out of a short-lived fashion for jokey abbreviations of deliberately misspelled phrases, and it then got a huge boost the following year when a political club adopted the same two letters for unrelated reasons. That double origin is part of why it stuck when the other abbreviations from the same fad died.

The competing proposals are not absurd, they're just unattested. That's the difference to hold onto.

Plausibility is the trap. Human languages are strange enough that almost anything sounds possible, which is why the field runs on citations rather than on stories.