Modern Marvels on the History Channel had a show called 'Super Steam' and it highlighted Jay Leno's steam engine collection. Apparently it refers to the governor on steam engines. The governor had 2 ball weights that would swing out as the engine's throttle was opened up more and more. At maximum speed, the weights would be all the way out, keeping the amount of steam being fed into the engine in check and preventing overspeed damage to the engine by an overzealous operator. Funny how sayings originate and evolve.
He also joked that since steam technology predated oil, he claimed that they used to slaughter pigs to use their fat as lubricant. I'll bet that smelled real good in a hot engine room after a while!
http://www.popularmechanics.com/cars/jay-leno/4213088