Groom
Middle English (in the sense ‘boy’, later ‘man, male servant’): of unknown origin.
wiktionary
1604, short for bridegroom(“husband-to-be”), from Middle English brydgrome, alteration (with intrusive r) of earlier bridegome(“bridegroom”), from Old English brȳdguma(“bridegroom”), from brȳd(“bride”) + guma(“man, hero”). In Middle English, the second element was re-analyzed as or influenced by grom, grome(“attendant”). Guma derives from Proto-Germanic *gumô(“man, person”), from Proto-Indo-European *dʰǵʰm̥mō; it is cognate to Icelandic gumi and Norwegian gume and, ultimately, human.
From Middle English grom, grome(“man-child, boy, youth”), of uncertain origin. Apparently related to Middle Dutch grom(“boy”), Old Icelandic grómr, gromr(“man, manservant, boy”), Old French gromme(“manservant”), from the same Proto-Germanic root. Possibly from Old English grōma, from Proto-Germanic *grōmô, related to *grōaną(“to grow”), though uncertain as *grōaną was used typically of plants; its secondary meaning being "to turn green".
Alternative etymology describes Middle Englishgrom, grome as an alteration of gome(“man”) with an intrusive r (also found in bridegroom, hoarse, cartridge, etc.), with the Middle Dutch and Old Icelandic cognates following similar variation of their respective forms.
etymonline
groom (n.1)
c. 1200 (late 12c. in surnames), grome "male child, boy;" c. 1300, "a youth, young man," also "male servant, attendant, minor officer in a royal or noble household ranking higher than a page; a knight's squire." Of unknown origin; no certain cognates in other Germanic languages. Perhaps from an unrecorded Old English *grom, *groma, which could be related to growan "to grow," and influenced by guma "man." Or perhaps from or influenced by Old French grommet "boy, young man in service, serving-man" (compare Middle English gromet "ship's boy," early 13c.). As the title of an officer of the English royal house from mid-15c. Specific meaning "male servant who attends to horses and stables" is from 1660s, from earlier combinations such as horse-groom, Groom of the Stables, etc.
groom (n.2)
"husband-to-be at a wedding; newly married man," c. 1600 (usually as a correlative of bride), short for bridegroom (q.v.), in which the second element is Old English guma "man."
groom (v.)
"tend or care for; curry and feed," 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.
