1 brain /ˈbreɪn/ noun
plural brains
1 brain
plural brains
Learner's definition of BRAIN
: the organ of the body in the head that controls functions, movements, sensations, and thoughts
often used before another noun
informal : the ability to think and reason : intelligence
often plural
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

informal or US beat/bash someone's brains in
: 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

: 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

: always in your thoughts

pick someone's brain/brains

: to talk to someone in order to get helpful information or advice

rack your brain/brains

see 2rack
2 brain /ˈbreɪn/ verb
brains; brained; braining
2 brain
brains; brained; braining
Learner's definition of BRAIN
[+ object] informal
: to hit (someone) on the head very hard