Slang: Space cadet

Definition: Someone who is generally unaware, or not paying attention, or out of touch with reality.

Example: 1) Suzie is a real space cadet -- I don't think she even knows what day it is.

Etymology: A 'cadet' is a student, usually at a military school, and 'space' is the place outside the earth's atmosphere, where the moon and the stars are. So a 'space cadet' is someone in training to float around in empty space, doing nothing but looking at the pretty stars.

fonte: www.peakenglish.com