Comes from dancehall reggae culture. Dubplate can refer either to an exclusive tune that only people with clout have access to or the acetate disk that those exclusive tunes are cut onto for play. Physically, a dubplate is a 10" or 12" slab of metal covered with an acetate laquer that is spun on a lathe and has music "cut" into it. This is actually the first step in manufacturing any vinyl record. If vynil is going to be pressed, the dub is then electroplated and the mold for the vinyl is cut from it.
Dubplate culture has been adopted by DrumnBass and UK Garage DJ's and producers as well.
"Give me a dubplate... I'll make your mind step down..."