异想天开

yì xiǎng tiān kāi

to have a wildly fanciful idea; pure fantasy

Character by character

异yì
strange

想xiǎng
thought

天tiān
sky

开kāi
open

他想不花一分钱就环游世界，真是异想天开。

He wants to travel round the world without spending a penny: what a pipe dream.

Tone: negative (criticism).

Usually dismissive: 别异想天开了, “stop dreaming”. Looking back on inventions that came true, people use it more kindly: 当年看来异想天开的想法，如今成了现实.

Learn this in Lesson 73 · HSK 6: 机器人与低空经济 (Robots, drones and the low-altitude economy)

A dialogue, a reading, word cards with audio and a drill that uses it, in Learn Chinese with Bao. The first lessons of every level are free.