WordWeb Online

1. Not practical; not workable or not given to practical matters
   "refloating the ship proved impractical because of the expense"; "he is intelligent but too impractical for commercial work"; "an impractical solution"
    
2. Not practical or realizable; speculative
   "impractical theories about socioeconomic improvement";
   - airy, visionary, Laputan, windy

See also: crackpot, crazy, half-baked, impossible, meshuga [N. Amer], meshugga [N. Amer], meshugge [N. Amer], meshuggeneh [N. Amer], meshuggener [N. Amer], nutty, practicality, quixotic, romantic, screwball, softheaded, unfunctional, unrealistic, unwieldy, utopian, wiggy [US], wild-eyed

Antonym: practical