If you say that something is insupportable, you mean that it cannot be coped with or accepted.
例句
Too much spending on rearmament would place an insupportable burden on the nation's productive capacity...过于庞大的重整军备开支会给国家的生产力带来难以承受的负担。Life without Anna had no savour, was tedious, insupportable.没有安娜的生活毫无情趣,单调乏味,不可忍受。