McGraw-Hill's Dictionary of American Idioms and Phrasal Verbs: All languages have phrases that cannot be understood literally and, therefore, cannot be used with confidence. 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 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.