Or when you are chapped between the balls and your legs/ass.
Simply put: Itchy crotch.
After a long day of swimming in the ocean it hurts far too bad to walk to your house or hotel. "Man do we have to go to the house? I'm critchin' really bad."
When everybody is wathcing you and you are wiggling to scratch the itch in your nuts. "Dude, make a distraction. I'm critchin'."
When picking your ass just doesn't do the job. "I'm not pickin my ass man, I'm hardcore critchin'."