I'm not sure where the whole "Yeah Baby!" thing started, but after a very interesting Matt Williams trip to the mound with Doug Fister earlier this week, Nats fans have taken and ran with the old 1997 Austin Powers made-famous phrase. During that mound pow-pow, Williams said he asked Fister if he wanted to face the next batter, to which Fister replied with the new catch phrase. And every game it grows.
