Dic: a pipe dream

• You could waste your whole life on a pipe-dream.
• Going to the West Indies is a pipe dream.
• We'll never have enough money. Your hopes of winning a lot of money are just a silly pipe dream.

N-COUNT | A pipe dream is a hope or plan that you have which you know will never really happen.

Fig. a wish or an idea that is impossible to achieve or carry out. (From the dreams or visions induced by the smoking of an opium pipe.)

.... Cobuild, McGraw-Hill Dictionary of American Idioms and Phrasal Verbs