Wordnet 3.0
NOUN (1)
1. 
 someone who seems unable to respond appropriately to reality (as if under the influence of some narcotic drug); 
The Collaborative International Dictionary of English v.0.48:
Space cadet \Space cadet\, n.
   a person who appears disconnected from reality, or living in
   his own world; -- sometimes used of people who are under the
   influence of mind-altering drugs. [derogatory]
   [PJC]
WordNet (r) 3.0 (2006):
space cadet
    n 1: someone who seems unable to respond appropriately to
         reality (as if under the influence of some narcotic drug)