when someone does something that is not acceptable, "Not cool man!" is usually an appropriate phrase to inform them that what they did was not nice, cool, funny, or tolerated by society, or just you.
The phrase of choice of a very loud Armenian male population, especially from Glendale, CA when they disapprove of anything not to thier satisfaction. Normally said in a very loud, booming voice with a rolling 'rrr'. This phrase is used mostly in driving situations where the all too perfect Armenian driving style has been disrupted in some way.
Shouting from his driver' side window - "Hey BRO! Not Cool Bro! Why you make lane change my lane when you KNOW I coming BRO!?? NOT COOL BRO!!".
An expression of mild exasperation or frustration - may be used as a substitute for a curse word at work or in front of children. (Inspired by the first Kid President pep talk).
Roomie #1: Where's the rest of the cake from last night? I was planning to have it for breakfast...
Roomie #2: I ate it after you went to bed...sorry.
Instead of sitting at your workdesk, you are sitting at the school library when opening a mail, link, picture, video etc. from a friend with explicit content. Either he warns you NCFS - Not Cool For School, or you ask him.