space cadet
US /ˈspeɪs ˌkæd.ət/
UK /ˈspeɪs ˌkæd.ət/
1.
a person who is out of touch with reality, often appearing dreamy, distracted, or eccentric
:
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He's such a space cadet, always daydreaming during meetings.
•
Sometimes I feel like a space cadet when I forget simple things.