noun
plural brains
[count] the organ of the body in the head that controls functions, movements, sensations, and thoughts
informal :the ability to think and reason :intelligence
informal :a very intelligent person
If you are the brains of/behind something, you are the person who thinks of plans or makes important decisions for a group.
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
blow someone's brains out
informal :to kill someone with a shot to the head :to shoot someone fatally in the head
cudgel your brain/brains
informal + old-fashioned :to think very hard :to rack your brain
on the brain
informal :always in your thoughts
pick someone's brain/brains
informal :to talk to someone in order to get helpful information or advice
rack your brain/brains
verb
brains; brained; braining
[+ obj] informal :to hit (someone) on the head very hard