Space cadet

[Ju]

1) What does it mean of "space cadet"

2) How can I make it in conversation? Shall I use it for blaming someone's mistake? May I have examples in usage?

Many thanks.

[mykwyner]

Back in the 1960's, a space cadet was someone who took a lot of mind-altering drugs and acted like it. Today, it means someone who is out of touch with reality to some degree (with or without the benefit of drugs).

"Allison thinks that her dog has psychic powers."
"She's a real space cadet."

[Tdol]

In the UK, it the drug-addled meaning is still fairly common.