What does Boxing Day have to do with boxing?
"long, narrow ulcer," late 14c., from Latin fistula "pipe; ulcer," of uncertain origin.
fistula fis·tu·la (fĭs'chə-lə)
n. pl. fis·tu·las or fis·tu·lae (-lē')
An abnormal passage from a hollow organ to the body surface, or from one organ to another.