noun: Marijuana of very high quality.
adjective: Amazing, really good, etc.
Definitely does _not_ mean pot with coke, as someone said before here.
I have heard that the word "chronic" meaning marijuana comes directly from the Dr. Dre album by that name, and this may be true.
However doctors also prescribe marijuana for "chronic pain," and people with chronic weed readily available to them often do become what experts call chronic (or habitual) users. So those could also be possible sources.
Yo, you got any chronic?
This weed is pretty good, but it aint the chronic.