In the fandom, "reverse harem" is a genre of manga and anime about one girl surrounded by a cast a male characters who are her potential romantic interests. Essentially, it's the harem genre, but with genders in reverse.
One question many people have is why the term sounds so stupid in English. A "reverse" harem? What's up with that? Why not call it a male harem? Reverse sounds weird.
That's because the English term comes the Japanese term for the same genre: gyaku-harem, or gyaku-hareemu 逆ハーレム.
This gyaku 逆 prefix can be translated as either "reverse" or "opposite." So it was a choice between "reverse harem" and "opposite harem."
Another anime term that follows the same pattern is one used toward violent scenes depicting characters being harmed: ryona リョナ, girls being harmed, and gyaku-ryona 逆リョナ, guys being harmed. Sumber https://japanese-with-anime.blogspot.com/
