A massive radio-cassette player, usually carried by stoned and thuggish black youths.
"Look at that cocky twat with his Brixton briefcase blaring out niggershit"
Refers to a DJ carrying either a crate or a satchel full of vinyl records - namely in Brixton - because of the popularity of DJ'ing in that area and the many genres of digital music that have come from that area.
Saw the DJ arriving out back carrying a couple of Brixton briefcases
A Mcdonalds/KFC burger box, as seen carried by every black male in Brixton
they all got on the tube stinking out the place with their Brixton briefcases