1brain 
/ˈbreɪn/ noun
plural brains /ˈbreɪn/ noun [count] 1 : the organ of the body in the head that controls functions, movements, sensations, and thoughts
▪ Scientists are learning more about how the human brain works. ▪ The left and right sides of the brain have different functions. — often used before another noun ▪ a brain injury/tumor ▪ brain damage/surgery ▪ brain cells/tissue 2 informal : the ability to think and reason : intelligence
▪ Don't be such an idiot—use your brain. [=head] ▪ She has a good brain. [=she is smart] ▪ I'm sorry—I don't know where my brain is today. [=I'm not thinking clearly today] ▪ If he had half a brain [=if he was at all smart], he would have left a long time ago. — often plural ▪ If he had any brains, he would have left a long time ago. ▪ She has both brains and beauty. 3 informal : a very intelligent person
▪ The other children always teased him about being such a brain. ▪ She is one of the best brains [=intellects, minds] in the field. ◊If you are the brains of/behind something, you are the person who thinks of plans or makes important decisions for a group.
▪ She's the brains of this organization. ▪ He was the brains behind the scheme.
▪ Scientists are learning more about how the human brain works. ▪ The left and right sides of the brain have different functions. — often used before another noun ▪ a brain injury/tumor ▪ brain damage/surgery ▪ brain cells/tissue
▪ Don't be such an idiot—use your brain. [=head] ▪ She has a good brain. [=she is smart] ▪ I'm sorry—I don't know where my brain is today. [=I'm not thinking clearly today] ▪ If he had half a brain [=if he was at all smart], he would have left a long time ago. — often plural ▪ If he had any brains, he would have left a long time ago. ▪ She has both brains and beauty.
▪ The other children always teased him about being such a brain. ▪ She is one of the best brains [=intellects, minds] in the field. ◊If you are the brains of/behind something, you are the person who thinks of plans or makes important decisions for a group.
▪ She's the brains of this organization. ▪ He was the brains behind the scheme.
beat/bash someone's brains out or US beat/bash someone's brains in
informal : to hit someone on the head in a way that causes serious injury or death : to beat someone very badly
▪ They threatened to beat my brains in if I ever came here again.
▪ They threatened to beat my brains in if I ever came here again.
blow someone's brains out
informal : to kill someone with a shot to the head : to shoot someone fatally in the head
▪ When he refused to hand over the money, the robbers threatened to blow his brains out.
▪ When he refused to hand over the money, the robbers threatened to blow his brains out.
cudgel your brain/brains
informal + old-fashioned : to think very hard : to rack your brain
▪ I cudgeled my brains for a solution to the problem.
▪ I cudgeled my brains for a solution to the problem.
on the brain
informal : always in your thoughts
▪ I've had pizza on the brain all day. [=I've been thinking about pizza all day] ▪ He's got sex on the brain. [=he's always thinking about sex]
▪ I've had pizza on the brain all day. [=I've been thinking about pizza all day] ▪ He's got sex on the brain. [=he's always thinking about sex]
pick someone's brain/brains
informal : to talk to someone in order to get helpful information or advice
▪ Do you have a moment? I need to pick your brain about a little situation that has come up.
▪ Do you have a moment? I need to pick your brain about a little situation that has come up.
rack your brain/brains — see 2rack
