Knock, knock

Firefighters in California last week called in helicopters to help "knock down the flames" of a wildfire near Oakland, a local TV station's website said. In parched Florida, where wildfires have burned for months, rains from Tropical Storm Barry "helped knock down intense flames," reported the Jacksonville Times-Union.

Can anything contain this hot new use of knock down? Sheila Roberge of Exeter, N.H., who's had her eye on the expression, says it's been spreading like you-know-what over the past five years. When did we stop "putting out" fires, she asks in an e-mail, and where does that knock downcome from?

She's right about recent history: as a firefighting idiom, knock down is far more common in the news today than it was five or 10 years ago. But it's been smoldering in our news archives since at least 1938, when The New York Times, reporting on a fire in an East Side brownstone, quoted Deputy Fire Chief Patrick Costigan: "Before the maid could be reached in a rear bedroom on the second floor it was necessary to 'knock down the fire' with powerful streams of water."

And that 1930s usage didn't come out of nowhere. Since the 15th century, says the Oxford English Dictionary, we've had both a literal knock down("knock to the ground with a blow") and a figurative one meaning "overcome, vanquish" (in an argument or a mental struggle).

Google Books turns up the usage in a 1782 letter to the novelist Fanny Burney from Samuel Crisp, her literary mentor: "In vain comes Voltaire, with all the powers of wit, satire, learning, and art, to knock down Shakespeare, and turn him into ridicule."

And as early as the 1800s, knock downtook on essentially the meaning firefighters use today, "to lower or reduce in amount." An 1846 report of the Journal of the Royal Asiatic Society of Great Britain and Ireland finds that "in May and June, 1843, the shipments of spice under the low duty were exceedingly large, so much so, that many expected the supplies would knock down prices."

Knock down didn't stop there. It also means, "to sell an item at auction" and "to earn" ("She knocks down $400 an hour"). It gave us knockdown furniture, the kind you bring home in a box and assemble with an Allen wrench and a plentiful supply of curses. Back when crooked train and bus conductors would skim from their passengers, they were said to be knocking down fares.

And knockis always happy, in the service of slang and idiom, to hook up with other adverbial partners. There's knock up, of course, in the sense now playing at a theater near you. (And at theaters in Britain, too. Though travelers are often cautioned that knock up in Britain means "awaken with a knock on the door," Lynne Murphy, linguist and bi-dialectal blogger, assures me that the "get pregnant" sense is no mystery to British audiences.)

Knock off has been popular, in various senses, for centuries. The American Heritage Dictionary of Idioms gives a few of them: He knocked off work at noon, from the mid-1600s; They are knocking off designer Swiss watches, late 19th century; They decided to knock off the old lady, early 20th century. Knock off was also a synonym, from a century ago, for the more familiar knock over, or "rob": The gang knocked off two liquor stores.

Performers have been knocking themselves out since the 1930s to wow audiences, or knock 'em dead (slang, 1889). Some, we've heard, like to knock back a few drinks (early 20th century), either before or after the big performance.

And in recent years we've heard knock-on effect, a Briticism meaning "secondary or cumulative impact," which has just a tiny toehold in America. Dell's plan to drop suppliers who are insufficiently "green," The Times of London said last week, "could have large knock-on effects for groups such as Intel and AMD."

Compared to this knock-on upstart, the firefighter's knock down is a venerable elder of English. But even if it weren't, it has an excellent reason to exist: A fire can be down but not out. Narrowly, knock down means "reduce the flame or heat on the more vigorously burning parts of a fire edge," as a Web glossary (seconded by Merriam-Webster) explains. As a San Francisco TV reporter noted last month: "An hour after they got it knocked down, they had a flare-up."

And despite its recent popularity, this knock down is an expression with a natural limit; it won't come up unless fire breaks out. True, if global warming turns swaths of the our planet to tinder, we may hear a lot more about knocking down fires. But if that happens, the spread of firefighting lingo will be the least of our concerns.

E-mail Jan Freeman at freeman@globe.com. For the Word blog, go to boston.com/ideas/brainiac/word.