Well there are several theories on this actually..I know a guy that did a paper on it for a philosophy class. The three leading involve George Washington and Thomas Jefferson and Wood panels, WWII and the adaptation of a French word or an ancient roman emperor. The WWII origins seem most likely. Surprisingly very few have to do with actual bull dung. If you really want to know there is a great philosophy book on the subject by Harry Frankfurt on "Bulls**t" It is worth the read!
"All I have to do is really learn how to dance with a bull. You just have to remember that he's definitely leading and you have to follow. You can't get behind at all. If you keep in time with him, you're all right. I just wish it wasn't such a hard dance to learn."- Fred Boettcher