Used when in a particular social environment (for example at the workplace, at school etc) there is nobody particularly fanciable, attractive or cute in your opinion but there is one person who is rather cuter and more attractive than the rest. He or she would be CBC.
Guy #1: Do you like anyone from your Maths class?
Guy #2: Well... I'm not crushing on anyone in particular but Jen's quite CBC.
Girl #1: But that's impossible! You don't fancy Matt OR Chris OR Daniel!
Girl #2: Well... now you point it out Chris is a bit CBC.
Person 3: CBC