When things turn sour, it means that they have become unpleasant or have soured, such as a relationship or a situation. Some synonyms for turn sour include go south, take a turn for the worse, go wrong, turn bad, deteriorate, sour, go off and degenerate. Each of these alternatives imply that a once good experience has transformed for the worse. For instance, a relationship may have gone south because of infidelity, a situation may have gone off due to mishandling, or a project may have degenerated due to poor planning. Knowing these synonyms will help you express yourself more accurately and effectively.