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Why can the meanings of idioms not always be understood from their individual words?

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Louisa May Alcott: Little Women — figure of speech Front cover of Louisa May Alcott's classic novel *Little Women*.

An idiom is a figure of speech that cannot be understood from the literal meanings of its individual words because it possesses a distinct, established meaning of its own.1

While the words within an idiom may have their own definitions, the phrase functions as a single unit of meaning that does not rely on the sum of its parts. For example, when someone says to "hold your horses," they are using an expression that means to slow down or wait; the listener understands this figurative intent rather than interpreting the words as a literal instruction regarding four-legged animals.1

Because these expressions are often figurative, they allow speakers to communicate concepts effectively by painting a mental picture, even when the literal interpretation of the words would be illogical or impossible. Consequently, understanding an idiom requires knowledge of the phrase as a whole rather than a word-by-word analysis.1