impractical/impracticable

impossible improbable infeasible unfeasible unworkable

abstract absurd chimerical idealistic illogical inapplicable inefficacious inoperable irrealizable ivory tower no-go nonfunctional nonviable not a prayer otherworldly out of the question quixotic romantic speculative starry-eyed theoretical unattainable unbusinesslike unreal unserviceable unusable unwise useless visionary wild won't fly

practicable practical probable realistic reasonable tenable viable