They are opaque or unpredictable because they don’t have expected, literal meaning. Even if you know the meaning of all the words in a phrase and understand all the grammar of the phrase completely, the meaning of the phrase may still be confusing. A phrase or sentence of this type is said to be idiomatic. This dictionary is a collection of the idiomatic phrases and sentences that occur frequently in American English. Many of them occur in some fashion in other varieties of English also
