"loud noise," 1560s, perhaps imitative. Klein compares Gaelic racaid "noise." Meaning "dishonest activity" (1785) is perhaps from racquet, via notion of "game," reinforced by rack-rent "extortionate rent" (1590s), from rack (n.1). But it might as well be an extended sense of "loud noise" by way of "noise or disturbance made to distract a pick-pocket's victim."[racket etymology, racket origin, 英语词源]