I've written about idioms before. I've explained them to my kids plenty of times. "That's an expression," I'll say. "It means this..."
English is full of little expressions that aren't literal, and a lot of these make reference to metaphors. I have a list of English examples in the post linked above and I'm not going to do a lot here, but take for example "I'm off to the rat race." That expression is all about metaphor. A person is a rat. Life, or at least work, is a race for rats. The metaphor then comes along with a whole set of implications about how the person feels about heading off to "the rat race," that it's pointless, exhausting, demeaning, etc. Depending on the character of the person who uses them, and how that person feels about work, for example, the implications of the expression may be interpreted somewhat differently.
This is an enormous opportunity for worldbuilders.
Some idioms might be cute, and some might be serious, but any way you approach them, they are incredibly illuminating of a culture and characters who belong to it. I personally feel that idioms are so closely linked to the culture of which they are a part that, if they are used outside their original cultural context, they stick out of a story when I'm reading it. If you're creating a world, you should be giving serious attention to idiomatic expressions.
One type of idiomatic expression is the aphorism - a phrase intended to give people behavioral guidance. "The early bird gets the worm" is used constantly in English, but this set of words, in this order, is so recognizable as belonging to our culture that I would hope I'd never run across it in a story world not directly linked to our own. If there are no birds, or there are no worms, you're in serious trouble. And even if there are, and your people place value on rising early or acting early, don't use it just as is. Change it. What are the primary motivators for your people to be getting up early, or acting fast? Create something that makes reference to that. Off the top of my head I'll give you this: "First arrow names the kill." This would be a society in which people hunt with arrows and whoever has their arrow hit first gets to receive some kind of honor. I'd work out the details with naming as I went. Story cultures can also have their own special values that will be honored with aphorisms. In Varin, the servant caste is guided by the expression, "Imbati, love where you serve." This is a big deal for members of the caste who have to struggle with their own identity and with cruel masters, etc.
Another type of special phrase arises around extremely common activities. In this context I think instantly of the phrases "log on" and "log off"... I mean, seriously. "Log"? I'm thinking this use of "log" goes back to the idea of a captain's log, but what you've got now is something where the expression is used so often that we don't really think about what the individual words mean, only what the phrase as a whole refers to. Because of the underlying connection to idiosyncratic activities of our own world history, this kind of phrase can't always be imported wholesale into a story world (hey, there's another expression!). Whenever you have a really common activity in your world (and it may not be common or have an associated idiom in ours), see if there's a special way people would refer to it, and how that might be connected to cultural details or cultural metaphors. I have used two different phrases involving the word "foot" in this context. In "Cold Words" (Analog Oct 2009) I had Rulii use the phrase "take foot" instead of "arrive." In my Varin world the servants don't "run errands" but "take foot assignments." This kind of tiny alteration can really help your world feel like it doesn't have to owe anything to ours, and can also create a wonderfully unique atmosphere.
I found myself listening in the other day on a forum conversation about a world that was using Chinese culture as its basis, and the writer was very concerned about whether to use Chinese idioms. Here's another very fascinating question. My own bias would be to say this: if your culture isn't actually a version of a culture, don't use actual idioms from that culture. Those idioms are going to broadcast the fact that this culture is at very least a fantasy or science fictional analog of Chinese culture (to use this example). Then if other aspects of the culture are non-Chinese, or if the language they use is not Chinese-derivative, the idioms will stick out by a mile. You can always alter or "translate" idioms. If you want to retain a Chinese flavor, one thing you can always do is have idioms play the same cultural role in your story world as they do in China. This is a link on the meta-level that won't actually require you to link your story world directly to China, but will give it some flavor that people will link with China. After all, one of the parameters of idioms is how often they are used and what they are used for.
I'll let you all think about this while I go off and take some foot assignments.
Note for Wednesday Worldbuilding fans: I have a couple of new entries that have come in, and I am planning to take them on, I hope within the next couple of weeks. Thanks for submitting!