Elbow grease is an idiom for manual labour and/or the process of working hard to accomplish an objective.
[1] The earliest evidence of the phrase in print was in 1672.
[2] Andrew Marvell, an English metaphysical poet, used the words in a satirical book about English parliament.
Marvell wrote: "Two or three brawny Fellows in a Corner, with mere Ink and Elbow-grease, do more Harm than an Hundred systematical Divines with their sweaty Preaching.
[4] In 1699, the phrase appeared in the New Dictionary of the Canting Crew defined as "a derisory Term for Sweat".