The phrase "icing the cake" is often used to describe adding a final decorative touch to something that is already completed or perfect. However, there are several antonyms that can be used to express the opposite idea. For example, one antonym could be "neglect," which means to disregard or not pay attention to something. Another antonym could be "sabotage," which means to deliberately ruin or damage something. A third antonym could be "destroy," which means to completely demolish or take apart something. These antonyms represent the opposite of "icing the cake" by suggesting that something is incomplete, imperfect, or actively sabotaged.