This abbreviation is most commonly used as a quick and simple way of telling an opponent on an online game that they are as rubbish as a Soggy, Saggy Ball Sack. This phrase can be extended by adding 'without any balls', implying that they are completely useless.
There is also an awesome clan on the XBOX 360 who use this abbreviation as their clan name. They use this simply because they created the phrase and also because they will destroy you at Team Tactical on MW2. This group of players also use the clan name 'Ouch' because you will feel extreme pain after meeting them.
'yeah, we won again'
'That other team were real SSBSs'
'I know, they were as useless as a soggy, saggy ball sack without any balls'
Rubbish team: 'Ouch! We lost again!'
SSBS clan: 'That's because you are as bad as a soggy, saggy ball sack without any balls'