The definitions of Groom are:

1 .A person employed to take care of horses or a stable.

2. A bridegroom. (A man who is about to be married or has recently been married.)

3. One of several officers in an English royal household.

4. Archaic

a. A man.

b. A male servant.

“Groom” (usage #1 and #4 above) at first meant “young man” or “male servant.” But because young male servants were likely to be put to work taking care of horses, the meaning of “groom” shifted over the years to mean a male servant caring for horses, and then, by extension, his duties. “Grooming” eventually broadened beyond horses to apply to any sort of hygiene or preparation, and even widened to other uses, such as “grooming” a candidate for office.

But that’s not the same “groom” as in “bridegroom” (usage #2 above).

The word “bride” has meant “woman about to be married, or just married” ever since it was first borrowed into Old English (as “bryd”) from the Germanic languages back around 1000 A.D.

The adjective “bridal,” did not originally just mean “pertaining to a bride or wedding,” as it does today. The “bride-ale” was the wedding feast (Old English “brydealu”), named for the vast quantities of ale served.

The original form of “bridegroom” was “brydguma,” combining “bryd” (bride) with the Old English word “guma,” which meant “man,” and which was a completely different word than “groom.”

Over the years, people stopped using “guma” by itself, but were still using the term “brydguma.” Eventually people began substituting the more familiar “groom” for the by-now obsolete “guma,” and “brydguma” became “bridegroom.”