"Brain" as Slang for "Oral Sex": fun with metonymy

[Jesse Sheidlower]

On Sun, Nov 07, 2004 at 11:09:36AM -0800, Arnold M. Zwicky wrote:
> On Nov 6, 2004, at 1:06 AM, Grant Barrett wrote:
>
> >Two more data points: "hanging brains" is to dangle one's scrotum
> >through the fly of one's pants and "to brain" someone is to hang one's
> >scrotum next to or on the face or head of a sleeping person, usually so
> >a picture can be taken.
>
> fascinating.  ya learn something new every day around here.

The first connection I heard between _brain_ and any part of
the male anatomy was _showing brain_ 'to inadvertently
display the scrotum, as under loose shorts', which I first
heard in the mid-1980s (and never heard thereafter). But
_hanging brain_ in the sense under discussion seems to be
widespread among men in the 30s, based on some completely
random polling in the last few days. Most of these hadn't
heard _brain_ 'fellatio', though this was universally known
among younger people (late 20s or younger) (of both sexes).

> what's the motivation for "brains" 'scrotum'?  is it based on the
> expression "have balls for brains" '[of a man] think with one's
> genitals'?  or on a perceived physical similarity between a testicle
> and a brain?  or what?

I'm very sure that _brain_ 'scrotum' is based on the similarity
of appearance between the scrotum and the surface of the brain
in conventional representations. This is different from _brain_
'fellatio', which is metonymically from _head_.

Jesse Sheidlower
OED