白日做梦 (báirìzuòmèng): to daydream

Recorded in the Ming-dynasty collection 《精选雅笑》 (Selected Refined Jokes) by Yuzhang Zuiyuezi: people who insisted they would pass the imperial exam by quizzing fortune-tellers were said to be 白日做梦 — literally 'dreaming in broad daylight,' i.e. delusional about a sure thing.

usage

Almost always used to put someone down: 你白日做梦! = 'In your dreams!' Stronger than English 'daydreaming' — implies the goal is impossible, not just distracting.