space cadetspace cadet
Slang a person regarded as silly, flighty, irresponsible, etc.
One who shows difficulty in grasping or responding appropriately to serious or necessary concerns; a spacy person: “the screwups and the space cadets—in other words, the fringe element” ( Linda Ellerbee )
(plural space cadets)
The original sense, which is not obsolete, is that of a person in training to be a (generally military) astronaut, especially in science fiction.