space cadet

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Slang One who shows difficulty in grasping reality or in responding appropriately to it; a spacy person: "the screwups and the space cadets—in other words, the fringe element” ( Linda Ellerbee).

Wiktionary

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

GNU Webster's 1913

1. n. 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.

WordNet 3.0

1. n. someone who seems unable to respond appropriately to reality (as if under the influence of some narcotic drug)