(verb) It means to relax. You say it to people to de-stress and unwind. It can be used interchangeably with "take a chill pill" or "take a load off" Or if a conversation is too heated and it needs to be cooled down, people will say "A/C!"
steve: WOAH! STFU! YOU'RE STUPID AND ARROGANT!
ben: NO, YOU'RE SUCH A BITCH FOR EVEN CARING!
mark: you guys, A/C. A/C.
Stephen King's book "The Talisman" has the lead character telling a man who's making a pass at him "No thanks, I'm strictly D C" to which the man replies "You think I'm A C?"