When we think of the phrase "sitting powder keg," we often visualize a situation or person that is incredibly volatile and ready to explode at any moment. However, there are other expressions that convey a similar sense of danger and instability. One term is "ticking time bomb," which suggests that a crisis is imminent and cannot be avoided. Another term is "livewire," which characterizes an individual who is unpredictable and potentially dangerous. "Unstable element" is also an apt description for someone or something that is precarious and liable to cause harm. Ultimately, these synonyms all convey a sense of impending danger and the need for caution and vigilance.