Add oil

In Chinese slang, the phrase "加油", literally translated as "add oil", means "put more effort" and is typically used as encouragement to try harder in order to succeed at something. I believe the origin come from the fact that we need to add oil (gasoline) to cars in order for it to move. As we are all expected to drive electric or hydrogen cars in the future, this might become an archaic slang in the not too distant future. 

Addendum: October 4, 2016
After reading this post, my wife asked me what the corresponding Chinese slang should be for electric vehicles. She said that "充電" which is the translation of "charging electricity" is not a good choice since "充電" is slang for "refresh" or "renew" and is typically used when one is tired or drained of energy.