Balls-Deep is the point when your testicles reach cold water while swimming thus making your testicles shrink into themselves.
the girl equivelent would be Clit-Deep
I was swimming in the Spokane River and I got Balls-Deep, holy crap it was cold!
when your magic stick is at max travel in your partners anatomy
i usually prefer to be balls-deep bout three times a day....only a sufer knows the feeling