Butt Hurt : Not Just For Breakfast!

I have lead a sheltered life. The one brief period of my existence that I was hip, NSYNC and The Backstreet Boys were still feuding for top spot on the walls of insane tweens. Therefore, I keep the Urban Dictionary handy, since I was weened on Reality TV, and I sometimes need the help with modern vernacular.
Last night, after our Sister's Class (Basic Fundamentals of Photography), my sister used a phrase I hadn't heard before, but, according to her, it was popular when she was in high school; Butt Hurt.
Out of pure curiosity, I decided to go to my awesometastic Urban Dictionary and look up this odd addition to my sister's everyday vocabulary.
Butt Hurt
Well... alrighty then.