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
"holy shit dude! leslie gave me some steamy brayayain on her front lawn after homecoming!"