Elbow Grease Meaning

Example : 1864, Henry Wells Holly, The Art of Saw-Filing, ch. 3, pg. 12:  It is astonishing what miserable saws are sometimes used by mechanics, or those claiming to be such. The only way they can be coaxed or driven through the wood is by having an enormous set, a liberal use of oil, and another lubricator called elbow-grease.1879, Anthony Trollope, William Makepeace Thackeray, ch. 5:  When we were young we used to be told, in our house at home, that "elbow-grease" was the one essential necessary to getting a tough piece of work well done. If a mahogany table was to be made to shine, it was elbow-grease that the operation needed.