The phrase "reptilian brain" refers to the oldest part of our brain that we share with reptiles. It controls our basic instincts and survival behaviors, such as aggression, fear, and the need for food and shelter. Full definition
Although it might seem like the prefrontal response to what is happening around us should be privileged, we would be severely disabled if we were not designed to sometimes have our mammalian and reptilian brains override our reasoning. (bettertherapy.com)