The word groom meaning "husband-to-be" is a shortening of bridegroom that first appeared in the early seventeenth century. It comes from the Old English word byrdguma, but the r was added because of the folk etymological influence of another word spelled groom that meant "attendant" (this sense is still around to describe people who care to horses, and comes from an unrelated Old English word meaning "grow"). Byrdguma comes from byrd, which was basically the precursor of "bride" and derives from a Proto-Germanic word for "daughter-in-law", and guma, the Old English word for "man". So, together, a bridegroom is just a "bride man", while a groom is just a man. Finally, guma comes from Proto-Germanic gumo, which eventually traces to a Proto-Indo-European word meaning "earth".
Hello! I'm Adam Aleksic. This year, I graduated from Harvard University with a degree in Government and Linguistics. There, I co-founded the Harvard Undergraduate Linguistics Society and wrote a thesis on Serbo-Croatian language policy, magna cum laude. In addition to etymology, I also really enjoy philosophy, trivia, vexillology, geography, board games, conlanging, art history, and law.