Jacob's heart fainted, or, was weakened, or failed, he fell into a swoon, as it is ordinary, because of the greatness and suddenness of the news, and the conflict of contrary and violent passions, raised hereby; grief at the remembrance of his former loss, and excessive joy for Joseph's recovery and felicity; hope that this might be true, and fear lest it should be but a fiction of theirs: any one of these passions are able to cause a fainting of the spirits, but much more when all meet together, especially in an aged person. He believed them not; partly because of the greatness, and strangeness, and desirableness of the thing; compare Psalms 126:1; and partly because they were by this very relation convicted of one lie about Joseph, in saying that he was dead, and therefore might easily be thought guilty of another.
Related Commentaries of Genesis 45:26
Genesis 45:26
26 And told him, saying, Joseph is yet alive, and he is governor over all the land of Egypt. And Jacob's heart fainted, for he believed them not.