RSM abbreviation for 1. (Military) regimental sergeant major 2. (Music, other) Royal School of Music 3. (Medicine) Royal Society of Medicine 4. (Automotive Engineering) San Marino (international car registration) [(sense 4): from R(epublic of) S(an) M(arino)]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014