An "I know this guy" story is a bullshit story where one person thinks up something funny, cool, gross, weird, or sexy, but essentially untrue, and tries to pass it off as true by framing it as if it happened to someone they supposedly know. It's similar to the "I have this friend" story, where someone asks for advice about a problem that's really about them but they're too embarrassed to say so. "I know this guy" stories are purely about entertainment, however, not about seeking advice.
Jerry: "I know this guy... he caught his sister having sex with their cousin and two friends in their uncle's bed."
Joe: "Sure, Jerry. You *know* this guy. Riiight."
Jerry: "I swear! It's true!"
Joe: "Bullshit. What was the guy's name then?"
Jerry: "Well I don't *know* him, but I heard about it."
Joe: "Sure you did. Sick pervert."
by kevnar February 19, 2007
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When you know how to personally contact an expert in a particular field--especially when the subject is unusual, strangely specific, or illegal. Often used when you do not want to reveal the identity of the contact or how you came to make the acquaintance of such a person.
Co-worker: "The boss said a hooker took off with the USB flash drive with all our vital code stuck up her vagina. How are we going to find her and get it back?"

You (not wanting to talk about how you did IT for a pimp/gynecologist who owes you a favor): "I know a guy."
by General Deviance June 28, 2018
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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.
by furrymonkey March 2, 2010
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