Short for Pacific Sunwear. A store that sells some pretty decent clothing. It's not too cheap but they have cute stuff. They sell more along the lines of skater/surf apparel. It doesn't make you a poser to buy from there, however. If the clothes appeal to you, then hey, buy them.
Girl #1: Hey, do you know where I can get some cute clothes from?
Girl #2: Well, you could go to Pac Sun.