U.S. Mil. a noncommissioned officer of high rank; in the Army, the rank just above sergeant first class; in the Air Force, the rank just above technical sergeant; in the Marine Corps, the rank just above gunnery sergeant
See master sergeant in American Heritage Dictionary 4
noun
a. Abbr. MSG A noncommissioned rank in the U.S. Army that is above sergeant first class and below the position of sergeant major.
b. Abbr. MSgt A noncommissioned rank in the U.S. Air Force that is above technical sergeant and below senior master sergeant.
c. Abbr. MSGT A noncommissioned rank in the U.S. Marine Corps that is above gunnery sergeant and below the position of sergeant major.