How do we know where a word really comes from?

We know where a word comes from by tracing it, like a detective, through old texts and by comparing it with related words in related languages. That's etymology, the study of how words began and how they've changed. Today it's a rigorous branch of linguistics, and its findings are often stranger than the myths.

Sound changes and meaning shifts hide a word's past. "Bless" is related to "blood": it originally meant to mark with blood. A "bead" was once a prayer; the word moved to the little objects people strung together to count their prayers. And Winchester, Gloucester and Tadcaster all carry a trace of the Latin castrum, "fort".

The hard part is resisting good stories. For a long time etymology was a game: the Greek poet Pindar made up word origins to flatter his patrons. The most popular origin stories today often have the feel of urban legends, like the tale that "snob" came from Oxford students writing sine nobilitate, "without nobility", shortened to "s.nob.". Sometimes the mistaken story even reshapes the word: "cockroach" comes from Spanish cucaracha, bent to sound like English "cock" and "roach".

One more trap: a word's origin isn't its "real" meaning. Arguing that today's usage is wrong because of what a word once meant is a known error, the etymological fallacy. Words are defined by how we use them now.