n. the tightening of one's butthole when one is shocked or frightened
rigor anal. adj. to have experienced the tightening of one's butthole when shocked or frightened
"Dude, why'd you break so hard?"
"The light changed faster than I thought."
"Well, it gave me rigor anus."
"So I was watching The Exorcist last night and it gave me the worst rigor anus"
"Whenever she announces a pop-quiz, I go rigor anal"