1. do or say something that may provoke an angry reaction in somebody
don't poke the bear by mentioning that
Whether always true or not, the idea of a bear as a fierce and violent animal is well established. This expression uses that stereotype to refer to a person who is highly irritable and therefore likely to react to even a slight provocation in an angry or aggressive way. It tends to be used more frequently in relation to somebody in a position of power or authority, as you are more likely to fear their reaction due to the consequences it might have for you. It is an informal phrase, often said humorously, and is common in both a social and professional context.