WordNet sense 1 (used of persons or their behavior):

insufferable, unacceptable, unsufferable

WordNet sense 2 (incapable of being put up with):

unbearable

WordNet sense 3 (totally unlikely):

inconceivable, unimaginable, out of the question

WordNet sense 4 (impossible to achieve):

unattainable

WordNet sense 5 (not capable of being carried out or put into practice):

impracticable, unfeasible, infeasible

WordNet sense 6 (certain to fail):

hopeless