The Bird = Extending the middle finger and "flipping someone off." Sign language for, "Fuck You!"
The "Word" = Originated in U.S. Prisons. Original meaning was, "my word is my bond", shortened to, "Word", meaning to, "speak the truth". See: Stephen Colberts segment on his show, "The Word".
Bird is the Word = Fuck You! Is the truth!
"Yea, if they knock on my door I will explain to them that, "The Bird is the Word". Fuck them and their military!"