The term “grease monkey” is an idiom, that is, it is not meant to be taken literally but is used as a comparative phrase. In common vernacular, the term is used as slang to refer to a mechanic who works on automobiles. It can also be used to reference a general mechanic who works on trains, aircraft or other machinery. The phrase also refers to a certain type of software extension to the popular Mozilla Firefox® Internet browser.
The term is not usually intended to be derogatory. Rather, it draws upon the general perception that, due to the nature of the job, a mechanic, and especially an auto or mechanic, will have grease marks or oil stains on his or her clothing. It also refers to a mechanic's ability to delve into small spaces and fix problems that might otherwise be difficult for an average individual to access.
Although the term does not generally carry a negative connotation, that wasn’t always the case. The phrase “grease monkey” likely has its origins in the late 18th and early 19th centuries, around the time when industry was growing in both Great Britain and the United States. During that time period, it was common and acceptable to employ child laborers to work around the industrial machines. Quite often, the children were sent to perform simple repairs and maintenance on the equipment because they were small enough to reach areas not accessible to an adult. The job often involved lubricating the gears, and was often a greasy, messy process.
The children who performed the jobs of lubricating and maintaining the machinery where recognizable by the grease and oil that smeared their faces and clothing. In order to reach certain areas of the machines, a child would often have to leap and climb. As a result, the child was often compared to a monkey. That comparison, coupled with the fact that a child’s clothing and face were often grease-stained, gave rise to the phrase, “grease monkey.”
Being a child laborer working on the machines in those early centuries was not an enviable position. When the term first originated, therefore, it was considered something of a demeaning slur to call someone a grease monkey. Over time, the negative connotations have faded and the term is generally considered harmless slang.
As an additional note, the term “grease monkey” also refers to a specific software extension to the Internet browser known as Mozilla Firefox®. The extension is sometimes referred to using one word, “greasemonkey,” but is also identified by the two-word term as well. The program extension, developed around 2008, is used to write and install scripts.