Colloquial term for the brain as a physical organ or tissue. Often used in casual conversation, sometimes with a slightly crude or humorous tone.
You have to really use your brain to solve this problem.
I hit my head so hard it feels like my brains are going to come out.
Zombies apparently eat brains.
My brain feels like it's melting in this heat.
頭脳 is a formal word emphasizing intellectual capacity or brainpower, whereas 脳みそ refers casually to the physical brain (or, informally, one's stock of smarts).
Standard colloquial spelling.
Ateji spelling using 味噌 (miso); rarely used.
Compound of 脳 (nō, 'brain') and 味噌 (miso, 'fermented soybean paste'), likely from a metaphorical comparison of brain tissue to miso paste. The exact origin is uncertain.