Elbow grease is an idiom for working hard at manual labour, as in "You need to use some elbow grease." It is a humorous reflection of the fact that some tasks can only be achieved by hard effort and human energy, contrasting with the idea that there should be some special oil, tool or chemical product to make the job easier.
It has also been used as a practical joke by a master tradesperson on apprentices, e.g. "Go fetch some elbow grease from him." Each tradesperson will say someone else has the elbow grease and send the unwitting apprentice on to another master tradesperson. The snipe hunt will continue until the apprentice gives up, catches on or runs out of people/places to search.