Backwards spelling of US ARMY. Acronym for the phrase "Yes my retarded ass signed up."
Also the title of a little known book, who's author constantly spams Myspace Army groups, in hopes that someone will buy his crappy piece of so called literature.
Person 1. "Did you read YMRASU?
Person 2. "Nope, I used it for toilet paper when I was in the field last week."
Person 3. "You actually bought it?"