Let's get down to brass tacks about idioms

Son of a gun. Idioms are actually nonsense yet hit the nail on the head.

We know what these phrases mean; we may even use them. But where do these funny and nonsensical statements come from? Since when can you know the ropes, knock wood or wag the dog?

An idiom generally is an expression different from its literal meaning. Often, only people in a particular region or class understand it. Some first appeared in the Bible or were penned by Shakespeare. And today, with the proliferation of mass media, they can spread thick and fast.

"Idiom" is a very loose term that can mean anything from the colloquial to a metaphor, says David Simpson, an English professor at the University of California, Davis.

"It's very rare we can track an exact origin for any of them," he says. "The remarkable thing about them is they spread very, very fast and the origins are lost almost immediately."

An idiom starts as a phrase, becomes an idiom when it catches on, and then dies as a cliché, says Robin Lakoff, a linguistics professor at the University of California, Berkeley. That happens faster in today's 24/7 media cycle, which spreads phrases in our language through television, music and YouTube.

For instance, a character on HBO's "Entourage" regularly says, "Let's hug it out." The phrase has been repeated in magazine articles and is entering common usage.

"That's our Shakespeare," says Lakoff. (Shakespeare is credited with idioms such as "wearing your heart on your sleeve.")

Today, comedian Stephen Colbert regularly plays off the literal meanings of idiomatic expressions.

"I stand by this man because he stands for things. Not only for things, he stands on things. Things like aircraft carriers, and rubble, and recently flooded city squares," Colbert said about President Bush during the 2006 White House correspondents' dinner. "And that sends a strong message that no matter what happens to America, she will always rebound with the most powerfully staged photo-ops in the world."

Susan Gass, a professor of linguistics and languages at Michigan State University, believes some of the long-used phrases stem from something real, but sometimes the particular reference disappears over time. For instance, a person who "sounds like a broken record" says the same thing over and over.

"Somebody born in the '80s or '90s probably knows what it means but doesn't really understand the concept when it originated," Gass says, because they might not ever have used records, which get stuck if there's a scratch. "It had some basis of reality when it first started to be used, but then the reality changes and the fixed form remains."

Some of the stories that explain idioms are simply that — stories, warns Richard A. Spears, a former linguistics professor at Northwestern University and author of "McGraw-Hill's Essential American Idioms Dictionary."

Although the intricate stories can sound plausible, often the simple explanation is the real answer, Spears says.

For instance, the belief that the phrase "Sleep tight, don't let the bedbugs bite" comes from the old practice of weaving ropes across mattresses simply isn't true, he says.

"It's a long, drawn-up piece of nonsense. ... It means simply to sleep soundly like you close the door tight," he says. There simply isn't evidence that proves people used square plates, which translated into square meals. "In that sense, square is the same as well-rounded or honest meal," Spears says. "Most of the explanations that are interesting are urban legend."

More interesting than the origins of idioms are the subcultures they encompass, says Simpson of U.C. Davis.

"It's a way people establish subcultures with each other," he says. "My kids, like totally whatever, are inserting themselves in a certain segment of the youth culture," he says.

So idioms and slang — such as the term "cool," which came from the jazz subculture — are born and spread within these groups.

It's like birds and the slight variations in their songs.

"I think language is the same," Simpson says, "Just another kind of birdsong."