In act 5, scene 2, Othello accuses Desdemona of giving her handkerchief (which was a gift from Othello) to Cassio and, by implication, of having had an affair with Cassio. Desdemona's initial reaction is shock and incredulity. She exclaims, "No, by my life and soul!" She then tells Othello to send for Cassio and to ask him if she gave him her handkerchief.
However, Othello at this point in the play is completely convinced of Desdemona's guilt. Iago's poisonous words have worked upon Othello and have overwhelmed Othello's capacity for reason. He refuses to listen to her protestations, and he implores her to pray for forgiveness so that when he kills her, her soul may yet go to heaven.
Desdemona seems to accept that she is going to die but still protests her innocence. She insists that she "never loved Cassio" and that she "never gave him token," meaning the handkerchief. To this repeated protestation of innocence, Othello responds with incredulity of his own. Convinced of Desdemona's guilt, he cannot believe that she still protests her innocence. He interprets these repeated protestations simply as further evidence of her mendacity. He says, "By heaven, I saw my handkerchief in's hand." He then reacts with sadness and despair. He still loves Desdemona and is overwhelmed with sadness that she should obstinately refuse to admit her guilt and that she should, thereby, damn her own soul. Despairing that she should do so, he exclaims, "O perjured woman!"