When someone says they are "sitting on a powder keg," it means that they are in a tense and potentially explosive situation. Some synonyms for this phrase could be "walking on eggshells," "treading carefully," or "on thin ice." These phrases all hint at the delicate nature of the situation and the need for caution to avoid a disastrous outcome. "Playing with fire," "lighting a fuse," and "teetering on the brink" are other synonyms that suggest the dangerous and precarious position that someone in this situation may find themselves in. Ultimately, all of these phrases convey the sense of imminent danger and the need for careful action to avoid a potentially explosive outcome.