A sore and tender area (sometimes bleeding) around the anus caused by moisture that has built up in the arse-crack. Usually a problem with runners and joggers where the combination of arse sweat and the constant rubbing together of the cheeks causes skin chaffing and irritation. Can also be caused by walking in wet clothes.
I went out for a run today as it was a nice hot sunny day. I was soon regretting it after an hour; I had a severe case of jogger's ringpiece and had to cut it short.