A common expression when someone is witnessing a gross, disgusting, revolting, or generally displeasing or uncomfortable action.
1st Dude: "Not Cool," a warning to the person starting the action
2st Dude: Rising anger, starting to understand what is happening
3rd Dude: Total outrage and bafflement, shock at what is taking place.
Kevin: Hey, I gotta show you something...
*starts unzipping pants*
Brad: What? Dude, DUDE, DUDE!!!
*Kevin pees on Brad*