Elbow grease is an idiom for working hard at manual labour

Manual labour

Manual labour , manual or manual work is physical work done by people, most especially in contrast to that done by machines, and also to that done by working animals...

, 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

Practical joke

A practical joke is a mischievous trick played on someone, typically causing the victim to experience embarrassment, indignity, or discomfort. Practical jokes differ from confidence tricks in that the victim finds out, or is let in on the joke, rather than being fooled into handing over money or...

 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

Snipe hunt

A snipe hunt, a form of wild-goose chase that is also known as a fool's errand, is a type of practical joke that involves experienced people making fun of credulous newcomers by giving them an impossible or imaginary task...

 will continue until the apprentice gives up, catches on or runs out of people/places to search.

Appearances in popular media

• The prank was used in the popular web series Red vs. Blue
  Red vs. Blue
  Red vs. Blue, often abbreviated as RvB, is a set of related comic science fiction video series created by Rooster Teeth Productions and distributed through the Internet and on DVD...
  
  where two of the Red soldiers tell their new recruit to go fetch "Elbow grease and headlight fluid" from the store. Their prank backfires when the recruit instead asks for the enemy's flag from the enemy's recruit, mistaking the other base for the store.