that part of the brain that senses danger, where instincts and gut feelings originate; primal thoughts; subconscious or involuntary processes; the amygdala
"Dexter" Season 5, Episode 7:
Lumen: I just had a feeling. Some sort of lizard brain thing.
Dexter: She's starting to sound like me.
"Dexter" Season 7, Episode 3:
Deb: How do you know he'll kill again?
Dexter: An alarm is going off inside my lizard brain.
"Fool Moon - The Dresden Files, Book 2" Chapter 15:
"I could have sworn I saw her face start to change, her bared teeth begin to grow into fangs. Maybe that was just the effect of the magic on my perceptions, though, or a primitive, lizard-brain sort of reaction to Tera rising to her feet and charging toward me with a howl. There was murder in her eyes. I hadn’t gotten beaten up twice, shot, and nearly strangled to get taken out by a misguided werewolf bitch."