Sitting watching either a movie, a play, a speech, a sporting event, or any other event that would have you seated and you are bored and in your boredom you fidget in your chair to find a suitable and comfortable position to continue not enjoying what you're viewing.
When wanting to inform someone in your party of how you are developing a seat rash it is best to do so in a whisper for everyone knows that whispering denotes importance and if you whisper you have seat rash you will have their attention and no one else will hear your private conversation. Though if they know their Urban Dictionary those that are watching will know what kind of problem you are having.
*whispers* This guy is giving me a case of the seat rash. When is he going to shut up? Better yet when are we gettin' out of here?
*whispers* This movie is such a seat rasher.