(1.) not cool to use, and considered especially harmful, self-destructive and low-class. The big three are meth, crack and heroin, and PCP is often put in this category as well. Others include synthetic weed and misused legal shit like mouth wash, Robitussin and Benadryl.
(2.) Of dubious or scandalous origin, i.e. product sold and misrepresented as good drugs when in reality they are shitty, usually cheaper stand-in stuff. Pills claimed to be 'ecstasy' can actually contain stupid amounts of other drugs, usually amphetamines, and "LSD" is often actually DOC.
(2.) Fuck dude, that tab was dirty! Shit tasted like iron, I'm gonna have a bad trip now, fuck. Never thought my contact would sell a dirty drug.