Expressão idiomatica adquire um significado figurado quando certas palavras são combinadas, o que é diferente da definição literal das palavras individuais. Por exemplo, digamos que eu disse: ‘Não se preocupe, dirigir até sua casa é moleza.’ Entenderíamos que a expressão ‘melzinho na chupeta’ significa que é fácil. Normalmente, obviamente não associaríamos a palavra ‘chupeta’ quando ele está sozinha a qualquer outra coisa que não seja chupeta. Mas, neste contexto, é um idioma bem conhecido.

Leia essa fabula com um expressão idiomática muito rica.

THE LION went once a-hunting along with the Fox, the Jackal, and the Wolf. They hunted and they hunted till at last they surprised a Stag, and soon took its life. Then came the question how the spoil should be divided. “Quarter me this Stag,” roared the Lion; so the other animals skinned it and cut it into four parts. Then the Lion took his stand in front of the carcass and pronounced judgment: “The first quarter is for me in my capacity as King of Beasts; the second is mine as arbiter; another share comes to me for my part in the chase; and as for the fourth quarter, well, as for that, I should like to see which of you will dare to lay a paw upon it.”
 “Hump,” grumbled the Fox as he walked away with his tail between his legs; but he spoke in a low growl—
        “YOU MAY SHARE THE LABOURS OF THE GREAT, BUT YOU WILL NOT SHARE THE SPOIL.”