WordWeb Online

1. Not practical or realizable; speculative
   "impractical theories about socioeconomic improvement";
   - airy, visionary, Laputan, windy
    
2. 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"

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

Antonym: practical