space cadet
noun
: a flaky, lightheaded, or forgetful person
Did you know?
Space cadet has been used derogatorily since the late 1970s, but long before then it referred to the rank that the character Matt Dodson hoped to achieve in Robert Heinlein's 1948 novel Space Cadet. Other writers of futuristic fiction followed Heinlein's lead, using the word in reference to young astronauts. From there the meaning broadened to cover any space travel enthusiast. Today the word is occasionally used as a slang word for a pilot who shows off, but it most commonly refers to those of us who may seem to have our minds in outer space while our bodies remain earthbound.
Example Sentences
Don't be such a space cadet.
Recent Examples on the Web Then Paul Rudd as Paris is a bit on the outside, and a bit of a space cadet, so an astronaut felt right. 
Liam Hess, Vogue, 2 Nov. 2021 Your high school English teacher might have called you a space cadet, but in reality, even the briefest mental vacations can restore a sense of well-being. New York Times, 10 Apr. 2021 By wielding a giant sword and chanting a mantra, Adora changes from mumsy space cadet into a disco-interpretation of a goddess: gold go-go boots, matching golden gauntlets, a winged vaguely Nordic tiara, and a crimson cape. Alex Abad-santos, Vox, 21 Nov. 2018 Congress would have to authorize a new branch, but Members of both parties seem willing, albeit after a fight over whose district will host the space cadets. The Editorial Board, WSJ, 4 July 2018 Pep Bonus trivia: The Kellogg’s Pep box featured cardboard cutouts of a space cadet cap, gauntlets and a ray gun. Jennifer Rude Klett, Milwaukee Journal Sentinel, 20 Feb. 2018
Liam Hess, Vogue, 2 Nov. 2021 Your high school English teacher might have called you a space cadet, but in reality, even the briefest mental vacations can restore a sense of well-being. New York Times, 10 Apr. 2021 By wielding a giant sword and chanting a mantra, Adora changes from mumsy space cadet into a disco-interpretation of a goddess: gold go-go boots, matching golden gauntlets, a winged vaguely Nordic tiara, and a crimson cape. Alex Abad-santos, Vox, 21 Nov. 2018 Congress would have to authorize a new branch, but Members of both parties seem willing, albeit after a fight over whose district will host the space cadets. The Editorial Board, WSJ, 4 July 2018 Pep Bonus trivia: The Kellogg’s Pep box featured cardboard cutouts of a space cadet cap, gauntlets and a ray gun. Jennifer Rude Klett, Milwaukee Journal Sentinel, 20 Feb. 2018 Word History
First Known Use
1973, in the meaning defined above