T-shirts proclaiming "Cool story, bro" take their statement from an Internet meme that began in approximately 2008. The phrase "Cool story, bro" is a sarcastic response to another person's story that is basically designed to defuse any argument and stop the speaker from continuing. T-shirts emblazoned with "Cool story, bro" are available for purchase on many websites.
The phrase "Cool story, bro" has been multiply sourced to both the anonymous imageboard website 4chan and to issue 122 of "The Incredible Hercules," a comic book published by Marvel Comics. Initially, the phrase was used as a sarcastic response when a gamer told a long, sad story about something that had happened in an online game. It is possible that the phrase had an earlier origin in the 2001 movie "Zoolander," in which a character responds, "Cool story" sarcastically after a long and rambling story is told.
The meme spread quickly in 2008 and 2009, often accompanied by images from the comic book. It took off even more when repeated on the reality show "Jersey Shore" as an expression of indifference toward the person speaking, and T-shirts began to appear as a result. In addition, a Twitter account was launched in 2011, retweeting stories worthy of the meme.