Space Cadet is an informal expression, referring to somebody who is light headed, deconcentrated, or has lost touch with reality.
People who are billed with this attribute often have irrational expectations, and seem too lighthearted and optimistic when a problem or obstacle is presented.
It is a derogatory expression too, so avoid using it in any professional environments as it can often be received as an insult.
The expression was introduced by author Robert Heinlein in his sci-fi novel Space Cadet. The book’s plot revolves around the character Matt Dodson, who aspires to be a space traveler.
Many authors began to use the expression during the following decades. Thanks to its popularity, people started to use it as a slang term too in the 1970s, referring to those who have their minds in outer space but bodies on earth.
The expression makes appearances in various science-fiction books and comics, however, these do not refer to the slang meaning of the phrase.
As a derogatory slang, it is primarily used in U.S. English, but generally it is not a very popular expression and is rarely featured in popular culture and media.