Code for when an adult/parent/teacher whatever is listening, watching, etc.
Person one: "Yeah she said she was going to hook up with-"
Person two: "9"
Person one: "..."
9 is a slang for 9mm handgun. If someone says they are gonna put a 9 in you, it means they are gonna shoot you.
1: What's up dickheads? Ugly fat idiots!
2: if you don't shut your fucking mouth I'll put a 9 in you!
A silent place of study and responsibility
a: Wanna go out drinking tonight?
b: Nah man I live in Kopstraat 9