The Fist means the AI is in war mode (WHEOOHRN or actually in a War) and will attack someone in the near future, which could be you. If you see it pop up, you will want to immediately assess your relationship with that AI and the factors that lead certain AIs to war with a rival. For instance, if Boudica was already Pleased with you before going WHEOORN, then she will not attack you. However, if the Fist appeared before she reached Pleased status with you, then you may still be a target. Each leader has a different personality and probabilities/limitations in this regard.
One thing to check is BUG Military Advisor screen...2nd tab (I think F5). There a colored bar next to each AI that basically gives the "temperature" of their status with each rival, including you. Orange to Red mean they will likely attack you, though not always.
Edit: In case you did not know already, WHEOOHRN = "We Have Enough On Our Hands Right Now". This is one of the possible status messages you see as well in the Diplo screen if you try to ask/bribe that AI into war with someone, but the option is red.