"Agg" derived from the word "aggro" from the skaters and surfers of SoCal meets it's roots at the World Famous SC Village. As the infamous team Hostile Kid's popularity grew, so did the usage of the word agg. It is now a widespread term used by all paintballers at least once.
Agg simply means cool. But in reality it means much more, agg is more than cool. Agg is not just how players dress or stand behind their bunker, it's also an attitude they bring to the field. A perfect example of this is Kenny Rosenberg (former Legacy now HK player) he brings not only fashion sense, but a magnetic attitude onto the field.
Some players dislike HK/Agg because of it's "overusage" or "poser fan following". Regardless, it is a word that will forever represent this era of paintballers.