Idioms are different from other expressions. An idiom is a phrase, sure, but as John Ayto puts it, this phrase “behaves more like a word.” 1 Unlike other expressions, the meaning of an idiom is not always easy to tell based on the words used. They are groups of words which mean something different than they appear to mean.
Although they are everyday figures of speech and we use them without thinking, they are almost always figurative. Despite this, we give no thought to them. We know what they mean just as we know what most words mean.
So much of what we say relies on grammatical rules. When speaking, we make choices based on a large number of rules, resulting in an astounding number of different word combinations. Idioms change this. They are pre-constructed (or at least partially pre-constructed) and set, and resist being analyzed by examining their parts.
Some idioms are more limited than other types of expressions in how they can be used. Words cannot be substituted, and tenses cannot be changed, for example, without changing the meaning. An example often given is kick the bucket. You cannot say the bucket was kicked and have it mean the same thing. These are called frozen idioms.
This is not to say that they are always, or even ever, completely unanalysable. Many idioms are at least partially compositional, and certain words can be changed with the idiom still being understood. For example, to open a can of worms means to do something that makes things more complicated than they were before, thus giving you more problems than you would have had if you had left things as they were. We can understand the “can of worms” as corresponding to “all the other problems.” And “opening” as corresponding to the action that brought about all these other problems. We might even say “open the proverbial can of worms,” thus underscoring the metaphor and calling attention to the fact that can of worms means the same things as many problems. So, idioms are not so straightforward and meaningless as many sources lead us to believe.
Throughout the site, when pursuing idioms, keep in mind that they can be very context-dependent. In fact, many idioms are only used in certain situations. The situations they are used in can give us clues to their analysis. For example, we’ve already seen that the idiom can of worms would be used in a situation in which one is dealing with a problem or set of problems. Likewise, we would never have to break the ice with our best friend. This idiom would only be used in regards to social tension or unease.
Idioms are sometimes confused with aphorisms, which are short sayings which convey a general truth, sometimes called proverbs. Just as often, idioms can behave like aphorisms and vice versa. They can also be slang.