When you see a beautiful woman and your dick gets hard, your jeans or pants also get hard around the groin area. It's another way of saying you just experienced a boner. A hard feeling in the jeans is a sure sign that you just saw something that you liked. This is caused by peeping on some big tits or catching a glimpse of a great ass. If the jeans are too tight, the boner may cause significant pain. The most comfortable way to wear a boner is either completely nude, totally naked, or with just a pair of lightweight baggy shorts on.
Steve: "Oh fuck, look at that fucking big-tittied blonde!"
Gary: "Shit man, she is one fine broad."
Steve: "Oh fuck, I just got hard jeans. I am envisioning slamming her right now."
Gary: "Her buttcheeks would be red when I was done with her."
Steve: "You want to meet up again tomorrow night and talk about fucking chicks?"
Hard Jeans can also be hard pants, hard shorts, etc.