lizard brain

Noun

lizard brain (plural lizard brains)

1. (informal) The most primitive part of the brain; the brain stem.
2. (broadly) Any part of a person's psyche or personality dominated by instinct or impulse rather than rational thought.
   • With her pistol drawn, she hurried down the ramp and into cover behind one of the landing pad’s walls, listening carefully. The hangar was as silent as—well, as a tomb. The temptation to take off her helmet was powerful, to rely on scents and sounds and instincts instead of equipment and sensors....2017 November, N. K. Jemisin, Mac Walters, chapter 8, in Mass Effect Andromeda: Initiation, 1st edition (Science Fiction), Titan Books, →ISBN, →OCLC, page 155

   Synonyms:

   id, reptilian brain

3. (broadly) Any part of a person's mind concerned with automatic, repetitive tasks rather than emotion.
   • Which was good. If she kept her superego occupied with minor considerations, the lizard brain could keep planning routes and obscuring caches and examining half-melted human candles for the Markey equivalent of spoor without interference. She didn't want her emotional intelligence and creativity to...2023 October 10, HarryBlank, “The Cruelest Fight”, in SCP Foundation, archived from the original on 31 Aug 2024
4. Used other than figuratively or idiomatically: see lizard, brain.