its a quote from the movie scarface which has become poular amoung people to say when they do things like expose thier penis or pull out a gun, or a baseball bat, a chainsaw, flip someone off, etc.
it is a way to imitate the persona of tony montana.
you girlfriend cheated on you and she dont know you know and you pull out a small sword and say hello to my little friend before you plunge it into the back of her skull as you "finish up" doggystyle.