space cadet

from 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)

from 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]