space cadet

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One who shows difficulty in grasping or responding appropriately to serious or necessary concerns; a spacy person.

from the GNU version of the Collaborative International Dictionary of English.

• noun derogatory 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun slang, derogatory One who deals with reality in a way consistent with being under the influence of (or "spaced out on") drugs.
• noun slang, derogatory One who forgets, daydreams, or otherwise is distracted from reality more often than most.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun someone who seems unable to respond appropriately to reality (as if under the influence of some narcotic drug)