A medium-sized (average 45 lbs.) dog developed over many years in Scotland for herding sheep. Widely recognized as the most intelligent of dog breeds, Border Collies excel as working dogs, but also dominate in sporting competitions like agility, flyball and frisbee.
Border Collies are typically black and white, but many colors and combinations exist. A BC may have a long (rough) coat, or a short (smooth) coat. Appearance is secondary to intelligence, temperament and working ability in determining if a dog is a "real" Border Collie.
(BCs have recently been inducted into the American Kennel Club for conformation showing. Many BC lovers oppose this because Border Collies have traditionally been bred for working ability with little regard for their appearance. BC advocates fear the breed will lose its working ability and intelligence if bred primarily for appearance.)
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.
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.