repercussion (n.)

early 15c., percussioun, "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" (from PIE root *per- (1) "forward," hence "through") + quatere "to strike, shake" (see quash).

In reference to musical instruments sounded by a stroke or blow, attested by 1776 (instrument of percussion). In medical diagnosis, "a method of striking or tapping the surface of the body to determine the condition of the organs in the region struck," by 1781.

The art of percussion, besides, although very simple in appearance, requires long practice, and a dexterity which few men can acquire. The slightest difference in the angle under which the fingers strike the thorax, may lead one to suspect a difference of sound which in reality does not exist. ["Laennec's New System of Diagnosis," in Quarterly Journal of Foreign Medicine and Surgery, November 1819]

early 15c. (Chauliac), "reduce swelling by repelling the humor causing it," from Latin repercusus, past participle of repercutere "to strike or beat back" (see repercussion). General sense of "beat or drive back" is from c. 1500. Related: Repercussed; repercussing.