The 'Bro! Not Cool' meme is a staple of internet reaction culture, used to highlight moments of social betrayal or a violation of the unwritten 'bro code.' While the phrase itself has been a part of slang for decades, it gained meme status through various image macros featuring characters with looks of pure disbelief or offense. It is commonly used in gaming communities, friend groups, and office settings to jokingly point out minor grievances like stealing a kill in a video game or eating someone else's lunch. The meme's popularity lies in its ability to address awkward social moments with a mix of humor and genuine critique, making it the go-to template for calling out 'uncool' behavior across social media.