A set phrase said in response to something that is considered unacceptable, inappropriate, or mean-spirited. A: "So then I pulled the chair out from under him just as he was about to sit down!" B: "Not cool, dude. You could have really hurt him!" A: "She said she was going to dock my pay because of what happened." B: "Wow, not cool!"
See also: not
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.