A square is a slang word for cigarette. The box that cigarettes come in ressembles a square. Hence the name, "square" for a cigarette. Smoking a square is the act of satisfying your craving for nicotine by smoking a cigarette.
"Let's go smoke a square."
"Hey buddy, do you got an extra square?"
The word mint is used to describe the condition of coins by a collector. A coin in mint condition is as perfect as the day is was made at the mint (The Place where coins are made). A heroin addict feels like pure ass until he or she shoots a bag of dope. When asked by my girfriend at the time how I felt after I shot-up 3 bags of heroin while being dopesick, I replied, "Damn, I feel fuckin mint." The shot of heroin instantly warmed my body and took away any of my withdrawal symptoms. I felt like I was in mint condition. Now, to a small group of local heroin addicts, the phrase "Let's Get Mint," means, let's go score some heroin and get high.
"Hey bro, I'm fuckin hurtin. Let's Get Mint."
"I'm dopesick as hell, Let's get fuckin Mint."
"Dude, that dope was fuckin mint."