c.1200, grome "male child, boy;" c.1300 as "youth, young man." No known cognates in other Germanic languages. Perhaps from Old English *groma, related to growan "grow;" or from Old French grommet "servant" (cf. Middle English gromet "ship's boy," early 13c.). Meaning "male servant who attends to horses" is from 1660s.
husband-to-be at a wedding, c.1600, short for bridegroom, in which the second element is Old English guma "man."
1809, from groom (n.1) in its secondary sense of "male servant who attends to horses." Transferred sense of "to tidy (oneself) up" is from 1843; figurative sense of "to prepare a candidate" is from 1887, originally in U.S. politics. Related: Groomed; grooming.