a really thin marijuana cigarette sometimes cut with tobacco. if an inmate smuggles some pot in the jail, he/she rolls it into a jailhouse joint and everyone who wants to smoke it with the inmate has sex with each other (including the owner of the joint). The ones in prisons can be huge, but in the county jail they're thin, hence the name. On the Outside, sex isn't necessary to smoke one.
dude: this aint no nick bag! I got ripped off! It's only half a nick!
buddy: at least you can roll a jailhouse joint outta it.