A statement of questionable legitimacy. There is usually little or no evidence to back up a stoner fact, however the maker of the statement will insist that his or her information is accurate and act as if it is common knowledge. The term derives from the suspect statements often made by stoners.
Josh: "Coughing gets you like ten times higher, man."
Spiegel: "Yeah, right. That is such a stoner fact."
Josh: "No, I'm serious, man, everyone knows it's true."
Spiegel: "Sure it is. Next you're going to be telling me that if my roommate dies, I'll get straight A's."