1) Impossible 2) Impossible to achieve 3) Unachievable 4) Unattainable 5) Undoable 6) Unthinkable 1) Impossible 2) Unachievable 3) Unattainable 4) Undoable 
incapable of being made actual or real, as an ideal or ambition: His dream of military glory was unrealizable. · incapable of being sensed or understood; unthinkable.