impracticable, impractical, infeasible, unusable, unworkable

debatable, disputable, doubtable, doubtful, dubious, far-fetched, fishy, improbable, problematic (also problematical), questionable, shady, shaky, suspect, suspicious, unfeasible, unlikely

implausible, inconceivable, incredible, unbelievable, unimaginable, unthinkable

futile, useless

absurd, fantastic (also fantastical), outlandish, preposterous, ridiculous