Joan Houston Hall, chief editor of the Dictionary of American Regional English, still remembers the day back in the late 1990s when she typed “scrid” into Google.

The word, meaning scrap or bit, was to be listed in the dictionary as a purely New England piece of vocabulary traceable to 1860. But suddenly there it was on the Web site of a lathe maker in California.

“I thought, ‘Oh no! This regionalism has jumped the country,’ ” Ms. Hall recalled recently in a telephone interview from her office at the University of Wisconsin-Madison.

She e-mailed the lathe maker, who wrote back saying he had learned the word from his girlfriend, who was from Maine. A “nice, tight regionalism,” as Ms. Hall put it, was saved.

Such was a particularly nerve-racking day in the life of one of America’s most ambitious lexicographical projects, which culminates with the publication by Harvard University Press of Volume V (Sl-Z) next month, a mere 50 years after the project was inaugurated by Frederic G. Cassidy, an exuberant Jamaican-born linguist given to signing off conversations with “On to Z!”...