Ballsack is identified by Wiktionary as an alternate form of
ball sack, meaning either a scrotum or an obnoxious person.
The word seems to be in occasional use in most English speaking lands. From a quick survey, I'd say it's most often written as two words when used of the male anatomy, and more likely as a single word when used as an insult.
Oddly enough it's not known by Green's Dictionary of Slang, but it is in the OED. The earliest occurrence known to the OED, as two words, was in 1974. For an objectionable person, it was first recorded in 1991. An example from the 2003 book
Jarhead: A Marine's Chronicle of the Gulf War:
I've been hearing funny rumors ... that the marine's family is more important than the Marine Corps. The ballsacks who say that are liars and possibly Communists.
I'll add it as a rare word.