1. A phrase to use as an excuse when you would rather not explain how you acquired something.
2. A phrase to calm your friend down after he/she has done something wrong and is looking for some help.
1. Jimmy: Dude, where did you get that gallon of PCP?
John: I know a guy.
2. Jenny: Oh my god, I'm pregnant! What the fuck am I gonna do?!?!
Jillian: Chill out, I know a guy.