Slang for heroin.
"Cinese Rocks" is also the title of a song co-authored by Johnny Thunders and the Ramones, and performed by both. According to an interview with Dee Dee Ramone sometime in the late 1980s, the band removed the song from their set list after Dee Dee kicked his own habit. I recall that Dee Dee's explanation was typically garbled, but it had something to do with the fact that the effects of heroin addiction on people's lives was awful, and they didn't want to make light of or glamorize it.
Judging from the writeup below, the band's attitude has changed. So has Dee Dee's; it seems he's performing the song in his solo act.
Update 2004: Dee Dee's attitude changed more drastically than I'd imagined. His habit came back, and he died of an overdose on June 6th, 2002.