What does Boxing Day have to do with boxing?
early 15c., "a striking, a blow; internal injury, contusion," from Latin percussionem (nominative percussio) "a beating, striking; a beat as a measure of time," noun of action from past participle stem of percutere "to strike hard, beat, smite; strike through and through," from per- "through" (see per) + quatere "to strike, shake" (see quash). Reference to musical instruments is first recorded 1776.
percussion per·cus·sion (pər-kŭsh'ən)
A method of medical diagnosis in which various areas of the body, especially the chest, back, and abdomen, are tapped with the finger or a plexor to determine by resonance the condition of internal organs.
in medicine, diagnostic procedure that entails striking the body directly or indirectly with short, sharp taps of a finger or, rarely, a hammer. The procedure was first described in 1761 by the Austrian physician Leopold Auenbrugger von Auenbrugg. Although generally ignored by his contemporaries, it is now routinely employed. The sounds produced by the procedure are helpful in determining the size and position of various internal organs, in revealing the presence of fluid or air in the chest, and in aiding in the diagnosis of certain lung disorders.