the fusion of these two popular uses of "brain":

1) blowjob; head; dome; oral sex
2) the brain-like appearance of a scrotum when pulled through a pant zipper to show to unsuspecting bystanders

coupled with a unique pronunciation of "brain" which incorporates dramatically oscillating vocals has yielded the following definition:

n. oral sex performed on the scrotum; gargling a nutsack; sucking on some balls