The sudden, acute onset of an intense itching sensation of the anus in which immediate action must be taken in order to gain relief. The sensation is compared to the feeling of a mosquito biting the rectum. This condition is also known as 'anal ants' if associated stinging and burning occurs.
I was saying my wedding vows in front of 300 people when I was suddenly stricken with a horrible case of rump chaw. There was nothing I could do but squeeze my ass cheeks together and pray it would go away.