What does Boxing Day have to do with boxing?
fibrillation fib·ril·la·tion (fĭb'rə-lā'shən, fī'brə-)
Fine, rapid twitching of individual muscle fibers with little or no movement of the muscle as a whole.
The formation of fibrils.
Vermicular twitching, usually slow, of individual muscular fibers, commonly occurring in the atria or ventricles of the heart and in recently denervated skeletal muscle fibers.