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A groom or stable boy is a person who is responsible for some or all aspects of the management of horses and/or the care of the stables themselves. The term most often refers to a person who is the employee of a stable owner, but an owner of a horse may perform the duties of a groom, particularly if the owner only possesses a few horses.
The word appeared in English as grome c.1225, meaning "boy child, boy, youth"; its origin is unknown. It has no known cognates in other Germanic languages (e.g. Dutch and German compound terms, such as Stal(l)knecht 'stable servant', or equivalents of synonyms mentioned below). Perhaps it stems from an Old English root groma, related to growan "grow" or from Old French grommet "servant" (compare Medieval English gromet"ship's boy", recorded since 1229). 
The meaning "man servant who attends to horses" is from 1667, although women and girls are often grooms. The verb is first attested in 1809; the transferred sense of "to tidy (oneself) up" is from 1843; the figurative sense of "to prepare a candidate" is from 1887, originally in U.S. politics.
The groom(s) usually clean stables (mucking-out), feed, exercise and groom horses.
A groom in private service is expected to be 'on call' during specified hours in case any member of the employer's family wishes to ride.
Grooms whose employers are involved in horse sports or horse racing are often required to travel with their employers to provide support services during competitions. The services required vary with the type of competition and range from simply ensuring that the horse is ready for the start of the competition to warming the horse up beforehand.
In competition, the term may have a distinct meaning. At a horse show, grooms outside of the ring perform standard grooming tasks, but if utilized inside the show ring are generally defined as an individual called in to assist an exhibitor with a horse while in competition. In combined driving the groom is the passenger and at speed is required to shift his weight to balance the carriage.
Ranks, synonyms and terminology
Stablehand is a more old-fashioned term; the variation stableman usually applies to an experienced adult, the lowest rank stableboy (corresponding to the first origin of groom) rather to a minor and/or trainee.
The historical synonym [h]ostler has meanwhile developed (in the United States) a new meaning of "rail employee".
Large establishments may employ several grooms under the management of the head groom or stablemaster. In many cases the head groom has complete responsibility for the horses including devising training schedules, choosing feeds for optimum nutrition and ensuring the horses are shod, wormed, inoculated and provided with timely veterinary care.
In African or Asian contexts English-speakers sometimes use the Arabic/Hindustani word sais or syce instead of "groom".