Answer:
I've looked all over trying to find the answer this question because its been bugging me since I watched the movie Fever Pitch. Apparently it came from a song in 1903 about a woman singing to her bird Tesse. The crowd started singing random songs and that is the song that helped motivate the Boston baseball, and it kinda became their theme song. read http://en.wikipedia.org/wiki/Tessie for more information if you are curious.