Many people who work in the horse industry, when asked of their occupation, will refer to themselves as a groom. So why is it called a groom? Oh, well because you groom horses. Surely that’s where it came from? But if I told you that you groom a horse because you’re called a groom, and then asked you why- would you know?
The word ‘groom’ first came into the English Language in the 13th Century. At this point, it referred to boys. With use, the definition broadened to all adults, but was then restricted to a male adult, with an inferior position in the house, effectively a servant. If you have a knowledge of the structure of servants – or watch Downton Abbey – you will know that there are several different servants, the names of their position referring to the job they had to conduct. For example, a lady’s maid was a maid who tended to the lady of the house.
By the 16th century the use of groom had narrowed to mean a servant who looked after the horses- similar to its use today. However the older use is still used in the British royal household, for example ‘Groom of the Chamber’
Source: ‘The Story of English in 100 Words’ by David Crystal