Dates to at least 1928. May have originated during the Industrial Revolution in Great Britain when children were used to grease the large rotating axles which were used to transfer power from one centralized steam engine to all of the machines on the factory floor. These children, covered in grease and crawling in the tight spaces in the ceilings, were equated with monkeys.
- (idiomatic, slang) A mechanic, often with the specific connotation of an automobile mechanic.
- I'm no grease monkey, but I can manage to change out my engine's spark plugs without assistance.