Do you know the difference between an expression and an idiom?
W.O.W. to the rescue!
Well, rescue may be too strong a word here, as there is some overlap between these two terms and you may be left scratching your head. But the most important distinction, in my book, is that an idiom is an expression whose meaning is more than the sum of its parts...in other words, the meaning of an idiom cannot be deduced simply by understanding the definitions of its components. Usage does strange things to words, and sometimes a word or group of words develops a meaning that makes no sense; it just is.
As an illustration of the difference:
"Lo and behold" is an expression that dates to the middle ages. "Lo" is a shortened form of "look," and so the expression literally means "Look and behold" and expresses surprise as it calls attention to something. (And please...don't write "Low and behold"!!) But both words means exactly what they say, even if not everyone is clear on what "lo" means.
"All of a sudden" is an English idiom. It means something different from what might be suggested by just a definition of the words themselves. It is an alternate way of saying "suddenly," but what in the world is a "sudden"? Its introduction by the indefinite article "a" brands it as a noun, one we don't ordinarily use in writing or speaking. And what of the "all"? (And don't even start with me about the abomination, "all of the sudden." I might have spasms. Instead I'll send you to this all of a sudden.)
When trying to understand the concept of an idiom, it's helpful to think of yourself as a non-English speaker learning the language for the first time. You run across the phrase, "Time's up," and you look it up in your English/Bornistanian dictionary and you read the definitions of "time" and "up." Do you now have an accurate sense of what the phrase means? I didn't think so. It's an idiom, and idioms are the bane of the student of almost any language. They simply have to be memorized and/or absorbed, not defined.
One of my favorite idiom anecdotes: We have a family friend from Lebanon who was hard at work on his English. He was aware of many idioms but didn't always use them appropriately. One day, after a particularly filling meal, he pushed away from the table and declared, "Oh, I'm fed up!"
Have an idiom that is especially amusing or baffling to you? Share it in the comments!