This is not slang. It is Dutch for "scrotum."
In the Netherlands, the pouch of skin and muscle that contains the testicles is called the balzak.
by TMD September 12, 2006
slang pronunciation of ball sack, or the scrotum, where your one, two, or three testicles tend to hang out, though usually in pairs of two.
I slapped her in the face with my balzak
by D. Will July 13, 2003