All that happened more that 2004 years ago, at time of writing, is considered BC. The date system comes from the spread of christianity and the desire to use the birth of Christ as the standard starting point for years. Anything happening after 1BC is refered to as AD (Anno Domini).
However, the monks that tried to work when the birth of christ was did so over 1000 years ago, and were missing some documents. These documents were discovered in the 20th century, and 'twas found the birth of christ to be four years before the dating system.
When someone says something to a member of their sexually preferred sex that is so obviously flirting. when this happens, anyone around that person normally shouts out BC.
The announcement of BC can often lead to cock blocking.
Guy: you look amazing (to a girl)
Friend 1: (insert name) is so hot!
Friend 2: I think you should try it on.
Friend 1: I'm going to lay down a cheeky BC.