A way for a drug addict or ex-drug addict to explain his extensive knowledge of drugs without actually admitting to being a serious stoner, crackhead, dope fiend, etc.
From "Super Troopers":
Officer Rod Farva: "Where'd ya learn that one, rook...drug school?"
D.A.R.E. officer: "Now kids, this is how you roll a joint. Never do this. Drugs are bad for you."
Your 8-year-old kid: "Bullshit motherfucker you don't know what the fuck you're doing. Let me show you."
(rolls a perfect fatty in about 5 seconds)
D.A.R.E. officer: "Wow son...where'd you learn that? You don't smoke marijuana, do you?"
Your kid: "Ahh hell no ya fuckin' pig...drug school...bitch!"
(lights up the J and smokes it)