Head in the clouds

Generated image of a man with his head in the clouds
To have your head in the clouds means you are distracted by thinking about dreams or ideas instead of paying attention to what is happening around you.

The expression comes from the image of a person's head floating high in the clouds, far above everyday life. It suggests someone whose thoughts are far away from practical matters.

He was walking down the street with his head in the clouds and almost missed his bus.