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

[Grant Barrett]

On Nov 7, 2004, at 14:09, Arnold M. Zwicky wrote:
> 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 wonder if there's a connection to the common belief that men tend to
think with their small head more than their larger one.

Grant Barrett