A shortening of "ghetto blaster," a very large radio or boombox that is usually positioned on one's shoulder. There are so many kids in the neighborhood with blasters that I can barely hear myself think!
ghetto blaster
slang A boombox; a large portable radio. Back in the 80s, the embodiment of the New York hip hop scene was the image of someone walking down the street with a ghetto blaster on their shoulder.
(ghetto) blaster
and (ghetto) box (ˈgɛdo blæstɚ and ˈgɛdo bɑks)
n. a huge portable stereo, often carried on the shoulder. (Associated with blacks.) Hey, turn down that ghetto blaster in here!