The Korean words sa and mul mean four objects repectively, and nori means to play. In the case of Samulnori, it refers to the four musicians playing and dancing with their four percussion instruments, which are kwaenggwari ,janggu, jing and buk.
(Source: http://www.lifeinkorea.com/culture/samul/samul.cfm?Subject=history)
Comments: kim68@indiana.edu